Digital tuning for old radios
A few years ago, I designed and built a circuit that would take the Local oscillator freqency from a Panasonic-National T-63 and subtract the 455KHz of the IF frequency, so the result could be measured by a common frequency counter and indicate the station being tuned directly.
I found this very helpful for shortwave tuning in the crowded international SW band on the T-63 dial.
The principle of operation is quite straight forward. The idea is to simply cut off the frequency output for 455 cycles every milisecond, so that the average frequency comming out is the same as the Station being tuned. This works because the frequency counter will not know the difference between the bursted version of the Local Oscillator or a continuous version. It is assumed that the frequency counter being used is the conventional kind, that counts cyles for a predetermined amount of time. This trick may not work with modern top-of-the line counters that measure more than cycles, they tend to also measure period.
I have used Frequency counters with Nixie digit tubes and they work very well, and look cool.
The attached schematic shows decade counters U1, U2 and U3. The 455 number is coded into the preset inputs D0-D3. You can change this number for different IF frequencies, like 262kHz, or 460kHz.
Let me know if you would like specific detail on connecting this to different kinds of local oscillators, including tube types, without disturbing the oscillator frequency.
- Subtract IF from LO (38 KB)
Very clever design! What do you recommend for a convenient 1 kHz time base? I suppose one could make a custom divider to operate off a higher frequency crystal time base, but I have a hunch you had something different in mind.
With today's nice programmable logic, I imagine one could pretty easily build a digital circuit to do all the counting and numerically subtract 455 kHz (rather than interrupting the input signal). Nonetheless, I like designs like yours that can be built with common logic chips.
Tom, thanks for the kind remark.
I cheated a little bit with the 1kHz source. As usual, I design from parts I have in my lab and, this time, I happened to have a handy 1kHz crystal oscillator at hand, in a 14 pin dip.
One easy alternative could be to use a widely available 32kHz watch/clock crystal with a CD4060 osicllator/divider chip, or something similar.
I have not built this circuit exactly has shown. I adapted this from a clock that I made, where it served as a 1s timebase. But this should work too.
The clock I built was made as an effort to save old nixie digital multimeters. The clock put out a resistance or a voltage that was a function of time. This resistance or voltage would then drive the input of an old nixie instrument and show time. Noon was indicated with 12.00kOhms or 12.00V. This way there is no need to rip out the nixies to build a nixie clock. If you like, I can send info about that. I think it is a bit off-topic for direct posting here.
I convinced my Linear Technology colleague Bill Walter to build a version of this clock that put out a frequency that is a function of time. This version is meant to re-use/save old nixie frequency counters. Bill's made his frequency output clock with a PIC, and it works great. He gave me one prototye that I use with an old HP5221B 5 digit nixie frequency counter. Bill has made a PCB THAT measures only 5cm square. I think he plans to make it available for sale. Bill also made a resistance output version with a PIC.
Keep in mind that the purpose of the Voltage, Resistance and Frequency clocks is equal parts humor and preservation effort...
Just as I encouraged Bill to make the new-and-improved version of my clocks, I encourage you to do the same with this digital tuning indicator.
Forum friends, the following two photos show the frequency counter adapter with the Pananasonic radio that I designed it for.
The frequency counter was built inside a metal box to eliminate digital noise radiation.
There is also an emitter follower wired inside the radio to buffer the local oscillator signal for the counter input.
The actual IC types shown in the photo are:
74192, 74LS192 x2, 74HC04 as preamp, 7402 x2. The hex inverter 74HC04 is used as a preamp, and should be CMOS logic, the rest of the logic can come from any logic family.