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 Silicon Chip Micromite DDS Signal Generator - explanation of sweep output screen
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 Return to top of page · Post #: 1 · Written at 10:11:06 AM on 19 May 2024.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

I have composed an explanation of the sweep output screen of the Silicon Chip Micromite DDS Signal Generator. Some features of this screen are hidden on the screen but explained in the text of the SC article on p66 of the September 2017 issue. My comments are distinguished with brackets from explanations from the text. Hope this will be useful if you are trying to get the hang of this very useful gadget.

Silicon Chip IF Aligner


 
 Return to top of page · Post #: 2 · Written at 10:42:04 AM on 19 May 2024.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

Hi STC I am looking forward to that!

I use the micromite extensively as a sweep generator and a separate CRO as the reader and calibrate as I simply could not relate the internal screen shot generated to reality.

Thats a problem with my brain function not of the micromite itself.
To me a resonant circuit should like one, not a strange derivative curve with markers that looks only vaguely like what is actually going on!

Limiting the micromite to being a IF frequency only also seemed odd to me as the sweep function from 0 hz to mega-hz is so useful.
Why design a single purpose function device?
With an external reader I can look at the frequency response of the RF tuning, IF , and audio circuits in a radio, in other words from front to back not just the middle!

Your work might help unscramble my brain and show me different.

Cheers, Fred.


 
 Return to top of page · Post #: 3 · Written at 11:32:48 AM on 19 May 2024.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

Getting to the screen in the attachment was frustrating involving blue sparks (from me). But it was worth it as it showed that my by ear IF adjustment with modulated 455kHz signal was surprisingly good, ony a kHz or so high. And this result was reflected in sensitivity and station separation that would be expected from three IF transformers. But like you I didn't figure out what the log button was for.

And it confirmed my initial instinct a year or so ago that the problem with the radio was wildly out of alignment caused by my ham handedness.

Now to sharpen it up with the CRO. Probably more blue sparks!


 
 Return to top of page · Post #: 4 · Written at 12:07:41 AM on 20 May 2024.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

Many manufacturers recommend re-alignment if the set has been tampered with: Bad alignment has always been top of the list for creating deaf sets and AGC / AVC being seriously helpful in messing up alignment.

If the oscilloscope can handle it I like Astor tend to use the Plate of the output tube for alignment as that can show distortion and other things not meant to be there.


 
 Return to top of page · Post #: 5 · Written at 8:47:45 AM on 20 May 2024.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

My set was originally tampered with because the IF transformers were moving inside their cans. I thought that this could lead to eventual breaks in the fragile wiring inside. Also, I wanted to inspect thetransformer and trimmer condition. The cans were taken off and the and the nuts securing the transformers to their bases tightened up. The condition was good but anumber of the wires had a number broken strands - one had only one strand making its circuit. These were fiddly but easy fixes.

One can was repaired at a time, and the radio turned on to see if all was working. Yes it was, but my mistake was to make a rough by ear alignment each repair to see what improvement could be made. These in total added up to a misaligned radio, and an oscillation problem which I think was because the IF frequency was at the oscillation point on the dial - around 750-800kHz. The radio was also completely dead below this frequency.

My immediate thought was I have made an inadvertant change that had introduced instability on the audio side. So a long look for such a cause - no luck. To cut a long story short bought the micromite gadget and attempted alignment. Feeding from the mixer did not work. So I decided to start t the other end of the IF train by feeding in 455kHz from the second IF valve grid and adjusting the 3rd IF transformer by ear. Immediate success. Worked forward to 1st IF and mixer valves and their IF transformers and had the set aligned. And sensitivity and selectivity restored.

I have tried the same strategy with sweep signals, but I can only get a good peaked sweep AGC response curve shape feeding from the mixer grid. Feed from 1st and 2nd IF grids produces a AGC flat response curve. Noisy too so that useable 3dB and midpoint frequencies are not obtained.

My feeling about this is that AGC is interfering. But what to do about it. The micromite sweep generator depends for its sweep response on the AGC. And a less noisy curve is obtained with a strong sweep signal and good AGC voltage. In line with your comments on AGC Marc.

So looking for a better strategy, probably involving a CRO. I don't have a way of introducing a marker, but at least I will get a look at the sweep response curve shape; the micromite does not produce a normal sweep shape as Fred says. This is because the X axis of the curve is log which compresses the curve.

As an aside, the micromite gadget has a mind of its own. It will change signal strength setting of it own accord to a default of 20, and will also change from a straight 455kHz output to sweep mode without command - frustrating if you are doing a by ear alignment as settings need to be remade. Also sometimes the hidden commands I have described in my attachment don't work.


 
 Return to top of page · Post #: 6 · Written at 8:46:45 PM on 20 May 2024.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

When sweeping any tuned circuit its best if the AGC is shorted to zero volts.
As you can sweep Aerial, RF, IF and audio stages you have to inject a level that runs the stage in question in a linear manner.
You inject a steady tone and get the levels sorted out first before sweeping.
Once you are sure the stage is linear then apply the sweep.
Yes its also best to kill the local oscillator so there is no interference.
I always find a way to inject the signal somewhere so there is no valve distortion.
The CRO is a reader and that is connected to the output of the tuned stage.
The CRO screen will then show the sweep frequency waveform with amplitude variation.
You can be conventional and hook the CRO in DC mode to the detector output DC for an IF sweep.
The CRO then shows a single trace line with resonant amplitude variation.
The time base of the CRO is adjusted according to the mode in which you sweep.
Locking of the time base to display one sweep across the screen can be obtained either internally in the CRO or sampling some signal and using the CRO locking mode adjustments to get a steady trace.
The sample and hold function in a modern CRO also makes it easy to "freeze" the scope trace either for file transfer or taking a picture with a camera.
I have used all these techniques in my many posts on this web site when explaining how I set IF trannies in projects or examined tuned circuits or audio response.
Really, I just use the sweep function of the micromite and its screen to do set up I never use the weird scope function .
In some of my projects I have used a RF generator to provide a marker pip just to show it can be done but really as the CRO has a known sweep rate of so many MHz per division the bandwidth of the sweep is read directly off that.
Fred.


 
 Return to top of page · Post #: 7 · Written at 11:52:21 PM on 20 May 2024.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

I use a decades old clone of an LSG-11, for general IF work. And it is used that infrequently it is semi permanently wired into an attenuator box that couples it to the counter and the attenuators within. The generator audio is fed to the ext. synch of the Oscilloscope, as that will give a more stable trace.

That also rids the bench of a mass of wires, as the coupling of and them, is done on the shelving above the bench, with most of the coax inside slotted cable channel along the leading edge of the shelf.

Marc


 
 Return to top of page · Post #: 8 · Written at 7:16:42 AM on 21 May 2024.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

Just an additional comment on AGC and sweep response.

There is an amplitude difference to the response curve if you leave the AGC active.
I prefer to disable any AGC on the signal valves.
AGC will compress the amplitude swing.

Because I am not measuring absolute values, using the CRO as a reader is not a drawback.
You can see the shape of the curve and detect any fault like ringing.
You would only need absolute values if you are comparing a coil response to a known target set of figures, that it is so many dB change for so much change in frequency. that is usually unknown with household radio sets.
Using the CRO just builds up a set of "normal" responses.

You can stagger tune IF stages to broaden the AF response and produce sharp or flat peaks as you watch the CRO and twiddle the slugs.

My mite is dead steady, does not change any setting in use.
Even if not used for months it powers up where I left it the last time.
I power it from the USB port on a CRO because that is convenient for me just a cable across to the CRO.
The external sense leads are coax with clips and I always put an external blocking cap in the centre lead.
I am always so careful not to get anywhere near a DC voltage in a radio, having blown the front end of a CRO by accidently touching a x1 probe on a HT rail! Replacing a cascade of Fets, chips and diodes is not funny.

A handy signal lead for the Mite can be a back to front CRO lead set to x10 or x100 and AC set if it has that.

The Mite is so useful as a general tool I'm amazed the designer has not done an article on using it this way and maybe introducing a marker system.

Fred.


 
 Return to top of page · Post #: 9 · Written at 8:56:01 AM on 21 May 2024.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

Thanks for all this Fred and Marcc

Response to Posts 6, 7 & 8 - thanks gents.

Fred:

-I was getting a good sweep output from the micromite with AGC unshorted with signal inserted at the mixer grid (osc. tran. shorted). No such luck with inserting at 1st and 2nd IF grids. But will try with AGC shorted - micromite article did not suggest this.

-will try steady tone suggestion and make sure there is no distortion

-my experience with using the CRO eg determining stage gain and measuring IF frequency is that these connections disturb the stage IF frequency slightly requiring retuning of the radio to the frequency that is tuned. I forsee the same problem doing sweeps. My low value mica cap gimmicks at the measuring point help greatly but don't eliminate the effect.Maybe Marc's attenuator box would help with this.

Thanks for the CRO tips - will get onto that soon. Have been rereading some of your previous posts. Glad to see that lack of a marker is not critical.

Marc:

-I hadn't forgotten your attenuator box set-up - perhaps even more important with a very senstitive set like mine. It become a rat's nest of leads with 4 micromite leads, analogue signal strength meter, digital meter on AGC. All of this is potential for stray signals getting in. And haven't connected the CRO yet.

Fred again:

Micromite article requires connection to AGC, so haven't tried that shorted, or fixed voltage applied. A complication is that detector output is split into two, for RF AGC, indicator and audio stage, and one for IF stages. Shorting it may give me decent curves on the micromite from the IF stages.

It was certainly my idea to flatten the curves and widen the audio bandwidth as stations are well separated and there is no monkey chatter.

My micromite was steady, the level and mode changes are a later development. Maybe it really needs to be in a metal box for shielding, though clip leads on the end of the coax leads would probably cancel such a box.

My inputs and outputs have always been well away from HT but it could have been zapped despite the input and output well protected - 600V on output and 4.7Mohm on input.

Don't quite understand "A handy signal lead for the Mite can be a back to front CRO lead set to x10 or x100 and AC set if it has that." AC? AGC??

A marker would be great - also a larger screen.

Thanks all.


 
 Return to top of page · Post #: 10 · Written at 8:39:10 PM on 21 May 2024.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

Hi STC, yeah I could have been clearer about the lead comment.

My Mite has coax connectors on the box.
Sometimes to pipe the signal out into a device I grab a CRO lead and use that as the delivery lead.
The body of the tip has the scaling resistors' or if an older one may also have an AC DC switch that inserts a blocking cap.
You are correct that the presence of the lead can disturb a circuit you are feeding in so on the X10 position the load may only be a few PF and a megohm or more and not disturb much.
Depends on how much signal you need to drive the stage, uV, mV or volts.

My workshop has many test leads on different benches for different CRO's, signal generators etc, so I just grab what I think is appropriate for the job.

You also have to be careful with the lead driving the reader CRO.
If connecting to a low impedance point, like a voice coil (ohms) or audio (Kohms) bare clip leads will do.
If going anywhere near a tuned circuit, watch out.
Sometimes I dont even directly connect, just clip a lead onto the insulated part of a signal wire.
The transfer is then fractions of a pf, at HF as in a tuned circuit this can be enough to see a good picture.
This is fine because you are looking at the SHAPE of a curve, not making an absolute level measure.
Horses for courses.

Fred.


 
 Return to top of page · Post #: 11 · Written at 10:03:50 PM on 21 May 2024.
Brad's avatar
 Administrator
 Location: Naremburn, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 7342

Document uploaded.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 12 · Written at 8:06:21 AM on 1 June 2024.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

Thanks for these tips Fred - never occurred to me to use CRO leads on the micromite - exactly the same bayonet connectors and built in extra protection.

I made the mistake of using exactly the same leads and alligator clips on the micromite so need to be vigilant - perhaps I wasn't and zapped it leading to current problems. Some heat shrink will distinguish them.

What mostly concerns me is being able to isolate detection of IF response to a particular transformer. Applying the sweep signal "lightly" through the insulation is a good idea. That way can apply it to the valve just before the following transformer. That is what I have meant before with the word "strategy". I couldn't figure out any other way of separating responses. OK to feed in a modulated signal at the start of the IF train an align by ear or signal strength meter - only a varying whistle to listen too. But not OK to feed in a sweep signal at the biginning of the train (mixer) and then start trying to modify sweep curve shape progressively down the line of transformers. At least it seemed to be a potential problem to me. My intention is to start at the last IF stage and work back individually to the first.

I have changed the IF from the specified 450 to 455kHz. Is this likely to be a problem with whatever IF/RF traps are in the radio?


 
 Return to top of page · Post #: 13 · Written at 9:03:45 PM on 5 June 2024.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

One does have to be careful not to get too carried away. Sets were in the main designed for a price, but many were actually designed to squeeze the best out of them.

Normally the fine wire in the IF only breaks when soldering. The main risk is corrosive breaks, to reduce that risk where there are dissimilar metals, one uses circuit board to coat the joint. If the former is not made rigid relative to the can that can cause capacitive changes, but its rare and many cans are lined to eliminate any contact or, arcing with or to the can.

If the coils are designed for 450kHz then to my way of thinking that is what they are set at. Because the aerial, oscillator coils and gang are a matched set and designed to track 450kHz apart. This to me, means you may be attenuating the signal. Working backwards on the IF is often needed when it becomes obvious that some uneducated Monkey tampered with them, in the vain hope of getting it to work.

I do not get carried away sweeping the coils in domestic sets unless there is a serious issue. These things are not HiFi. The basic steps are to set the mechanical dial tracking first. If its been messed with & set wrong the trimmers / slugs, may not have the range to correct the error.

As many have found out, without touching a set AGC / AVC is the enemy of alignment and most Pentagrids. SW is good there as it is usually disconnected from the Pentagrid. My Signal Generator has no DC blocking & puts out volts. Hence the capacitive divider to DC block & cut the signal amplitude down. As noted: The Counter, Generator and an Oscilloscope connect together on the grounded metal box. All of that connecting is shielded cable, even the audio when used as sync.

Having the oscillator running is a mixed blessing, Its mainly Auto-dynes (Screen grid) sets with a separate exciter tube that say remove it to kill the oscillator. RF traps are more common in "Infinite Impedance" detector circuits and the Plate circuit of TRF stages before a superheterodyne mixer. In the odd set I have added a radio frequency choke to get the RF out of the audio as some tubes are not all that fussy as to what they amplify. One of the reasons full volume is suggested, is that you may need to here the signal that you are injecting.

Whilst only the CRO may be able to read it Markus & Levy suggest 5-12uV into the antenna terminal (that's usually for setting 600kHz for the oscillator where noted); 50uV on the modulator grid; 3,500uV IF grid. Interesting that I have seen no info as to that being peak to peak.
I have noted on the US Forum, using a battery to tie the AGC down.

I often use the IF calibration to see if the signal is getting through to the speaker & the CRO can be used to find where it died, should that happen. Astor often had a dedicated plug attached to the plate of the output tube for alignment. That allowed peaking IF's using low AC volt range.

Note: Meters of the day like like Peak 200H often had a dedicated plug for "output"(audio method of alignment). With the Peak it had a series 0.1μF HV cap. If you have any meter of the era with that, its likely waxed paper & like the one in the Peak, a dud: Replace. KV rating recommended.


 
 Return to top of page · Post #: 14 · Written at 9:33:35 PM on 5 June 2024.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

The suggestion to change the IF from 450 to 455kHz came from one of Peter Lankshear's articles where he was talking about an STC radio, suggesting that the change to 455 can be made. So I went by that as I had noticed the odd whistle in the BC band. No whistles at the moment and performance is quite good regarding selectivity and sensitivity.

I agree re these radios are not hifi, and each coil would have been peaked at the IF with no attempt at producing a flat IF response curve. And the average serviceman would not have had the equipment to do it. So I have decided to leave well enough alone as I have cured the complete misalignment. And especially as my micromite gadget is u/s. Another story - may write that up.

I agree no real need to sweep - just getting to the bottom of the complete misalignment caused by my rough misalignment by ear as each IF coil was repaired to fix frayed wires and transformers loose in their cans - the cause of the fraying.

Will get on with other issues, especially a vibration that has developed in the speaker.


 
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