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 A Sweep generator design for 400 to 500Kc IF's with octal valves.
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 Return to top of page · Post #: 1 · Written at 9:27:37 AM on 8 September 2016.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 883

Here is the next step in the development of a Test Generator for superhet receivers to show the IF response.
I have stuck with the theme of using WW2 octal valves and finding out the principles of using valves to do things that would be so easy to with a PIC chip! I have a box full of metal valves like 6SH7 and glass types like 6SN7 so will use those.
I like these valves, you can accidently short the HT to the grid and the worst that happens is something may smoke a bit or go a bit red and then recover when you remove the stupid connection. Not like accidently shorting something on a PCB and having to replace a long train of chips with 40 pins or worse still unmarked SMD parts!
As usual I will email in a paper as a PDF on the building and first use of the generator.
Cheers, Fred.

Sweep Generator 400 to 500kHz

 Return to top of page · Post #: 2 · Written at 7:23:09 PM on 9 September 2016.
Brad's avatar
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6354

Document uploaded.

A valve a day keeps the transistor away...

 Return to top of page · Post #: 3 · Written at 4:14:32 AM on 10 September 2016.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 766

I did something somewhat similar, though only using solid state equipment:
To see the passbands of various ceramic and other filters, and it turns out I have the equipment to make it happen. A frequency generator (VC2002 function generator), a sweep generator (Heath 1274 sweep function generator, and a scope (Tek TAS465). The setup:

I can adjust the amplitudes on both generators. I can see the shape of the bandpass (linear, not dBs though) and I can also see the amplitude of the marker frequency outside the passband of the filter on the scope. The scope is triggered from the sweep gen's "pulse" output (one pulse per sweep). The sweep is a frequency chirp, here from about 410 to 470kHz. The marker is always on. But the marker has to go through the filter to get on the scope, and changing the marker frequency I can see when it drops to half amplitude and thus the 3dB points, high freq and low. The marker also beats with the sweep generator (which doesn't really show in the pictures) when the frequencies of the two happen to coincide.

The Device Under Test (DUT) is a ceramic filter at 455kHz, and has a little more than 20kHz bandwidth at 3dB. Which would be good for 10kHz of AM radio station audio, the max the FCC allows. Hifi AM. Smile

It helps a lot to have a marker frequency of known frequencies so you can figure out the bandwidth and if you have the center frequency right. You could get some crystals like 445kHz, 455kHz and 465kHz and build a crystal oscillator in your device.

This setup ( and yours too) is invaluable when trying to tune up an IF like this below:

The buffer transistor helps make up for insertion loss. Also allows me to terminate the IF transformers with the same impedance that was used in the circuit they came out of.

 Return to top of page · Post #: 4 · Written at 8:16:21 AM on 11 September 2016.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 883

Hi wa2ise thanks for the pix and thoughts that all gives me ideas. I can couple a digital signal generator loosely into the valve sweep and put a marker onto the response in the same way as you have. With the digital readout of the DSG that gives me an accurate frequency reading when I need it such as setting the center frequency of the IF accurately.
As it turns out I get a sweep of 5Kc per division from my valve sweep set up showing on the CRO face at a particular control setting and just leave that set as is and the response window is then about 40Kc wide and perfect for displaying the band pass. I have now added more circuitry to the sweep generator which is a 6SK7 IF valve and a 6H6 detector and AGC rectifier this then mimics what is in a superhet at the last IF position and a test IF can then be tested in its natural habitat.
I tested a bunch of IF transformers from all ages and could see the progression of design. Circa 1930 transformers are lower gain and very sensitive to their surroundings with the response curve varied greatly by the presence of the shield can or even being handled and whether the can is grounded to the chassis. The response curve may also vary with signal level and you can see the shape squirm about assymetrically as you go from zero signal level to a maximum. You don't think of these things as most IFs are just bolted to the chassis and you just adjust them and move on. Later IF's are much tighter coupled and less affected by the surrounds and the operating conditions. 1960 full size and miniature IF's have symmetrical skirts and the response shape stays the same at any operating condition.
I'll document the update to the generator with the amended circuit and add this to the thread.
Cheers, Fred.

 Return to top of page · Post #: 5 · Written at 10:12:08 AM on 14 September 2016.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 883

Hi All, here is the latest from of the generator as an article almost like a "you can build this" project with a circuit and parts list.
If any body wants to build this a discarded small mantle radio chassis would be a perfect base as this gives most of the parts already there such as the power supply, the valves, the tuning gang and oscillator coil. You would pretty much just remove the output valve stage and convertor and wire in the sweep valve, put a front panel on it, mark up the controls and terminals and you are home and hosed!

Sweep Generator For Testing IF Coils

Cheers, Fred.

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