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KE1GF's 455 kHz => 11 kHz down converter
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KE1GF's 455 kHz => 11 kHz down converter
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Bill (KE1GF) provided me with the schematic for a 455 kHz => 11 kHz down converter that he designed. With his permission, I am posting (here) the schematic and some information about it.
 
Posted below are:
 
1. Bill's (KE1GF's) schematic for a 455kHz => 11kHz downconverter
2. My PowerPoint(R) version of Bill's schematic.
 
Note that I added a dc blocking capacitor between the output of the 74161 and the input of the SA602 because the manufacturer's specification sheet for the SA602 asks that this be done
 
Note that the preferred 455 kHz peak input voltage to the SA602 mixer is approximately 10 mV. You may need to adjust the 33k ohm resistor so that the 455 kHz input voltage divider produces the proper input level to the SA602.
 
All of the part's on Bill's schematic are available from Digi-Key.
 
When you order the ECS programmable oscillator (P/N: ECS-P145-B-ND) the Digi-Key sales person will ask you to tell them what frequency you want them to program the blank part to. The frequency you want to ask for is*:  (your i.f. frequency + 11 kHz) x 8.  For an i.f. frequency of 455kHz, this corresponds to (455kHz+ 11 kHz) x 8 = 3.728 MHz.
 
The part number for the 74161 is: 74F161APC-ND. The part number for the SA602 mixer is: 568-1201-5-ND. All of these parts mount in DIP sockets. The SA602 is an 8-pin dip. The 74161 is a 16-pin dip. The ECS programmable oscillator is a 14-pin dip. All of these parts will work off of a single 5 volt supply voltage.
 
*Alternatively, ask for the following frequency: (your i.f. frequency - 11 kHz) x 8. 
 
Which frequency should you specify? It doesn't matter all that much... but, depending on how your receiver is designed, your downconverter will shift r.f. signals, that are above the r.f. frequency that your receiver is tuned to, either to: i.f. frequencies that are above 11 kHz or i.f frequencies that are below 11 kHz.
 
For example, on my 75A-4, r.f. signals that are 1 kHz above the r.f. frequency that the 75A-4 is tuned to show up 1 kHz below the 11 kHz i.f. center frequency. If I had picked the alternative programmable oscillator frequency choice,  then r.f. signals that are 1 kHz above the frequency that the 75A-4 is tuned to would show up 1 kHz above the 11 kHz center frequency.
 
Either way is okay, once you get used to it... but one might be more convenient than the other.
 
 
Of note:
 
For displaying the spectral content of the 11 kHz i.f. output of Bill's circuit, I use a freeware application called Spectrum Laboratory... that you can download from the Web.
 
a. Set your computer's sound card's sampling rate using the Spectrum Laboratory program. You set this by: pulling down the "options" pull-down window, then clicking on "spectrum display setting", then clicking the "audio I/O" tab, and finally selecting 48000 for "audio processing sample rate".
 
b. Set the frequency display range to be around 5kHz -17kHz (or what ever range you want, centered at 11kHz)
 
c. Set the dB range to what ever it needs to be to include both the carrier and the sidebands (I'm using -10dB to -100 dB at the moment).
 
If you haven't used Spectrum Laboratory... you will have to play with it for an hour to learn how to set it up so that it displays only the things you want to see.
 

455khz11khzdownconv.jpg

11khzdownconverter.jpg

w1ux.jpg

Above is a "screen shot" of the Spectrum Laboratory program working on a 5-year-old laptop computer with its internal sound card... showing an off-the-air spectrum and associated spectrogram of an AM signal. The receiver I used to make this measurement was my FT1000 MkV with a pair of Collins 6 kHz mechanical filters in its two i.f. filter slots. Thus, the spectrum falls off very rapidly +/- 3 kHz from the 11 kHz i.f.center frequency. To make this measurement, I intentionally tuned the FT1000 MkV to be 2 kHz off from the carrier "zero beat" frequency. Thus the AM carrier appears at 9 kHz. The spectrogram shows about 80 seconds of measurement time... where the top of the spectrogram corresponds to the present time and the bottom corresponds to 80 seconds into the past. Note that the light green color illustrates where most of the sideband power is concentrated. The abrupt change in the spectrogram below the dotted line (about 60 seconds into the past) corresponds to when I was tuning the signal in on my receiver.