More sophisticated (read more stable and actually usable for ham radio) short wave receivers, such as those using superheterodyne circuits, can be quite complex, especially for someone (me) who has not had much experience with radio frequency circuits. I have often thought that what I would like to try is a project that involves a series of “modules” so I could build each subsystem and work out the kinks before tying all together into a working receiver.
Lo and behold, I found a publication by Frank W. Harris (titled Crystal Sets To Sideband, 2002.
Harris describes a number of separate modules/circuits for each of the system boxes above.
VFO prototype works, but a neater and more stable one will be built eventually.
12/10/2020: Audio Amp is complete and works - although this is probably the easiest module because we're not dealing with high frequency signals.
Variable Frequency Oscillator (VFO) to be used as local oscillator in the receiver
I built a number of VFO circuits to try to get one that was as stable as possible. This last one seems to fit the bill. The two jacks in the upper right part of the enclosure lid are for the 5VDC input. The connector to the left of these is for the RF output (approx. 4.500 MHz to 5.500MHz). The variable air capacitor (15pF) on the right is for fine tuning the VFO frequency, while the one inside the enclosure is for rough tuning.
The Intermediate frequency (IF) Amp layout sketch and finished circuit
Assembling the modules for testing: Left to right... 5volt precision power supply for VFO; VFO with lid removed; Mixer (BFO is also in that module but unused right now); IF Amplifier; Product Detector and Audio Amplifier (without enclosure).
Testing Setup: Signal Generator (above oscilloscope) tuned to 4.00MHz feeds a signal with a tone into Mixer, along with 5.00MHz signal from VFO. The result should be a 9MHz output (along with a 1MHz signal we don't want). The IF Amplifier should increase the 9MHz signal and output it to the Product Detector that is supposed to detect the audible tone, which should output that tone signal to the Audio Amplifier then to the speaker so we can hear the tone.
VFO output is 5.00MHz (left) and Signal Generator output is 4.00Mhz (right)
Initial testing
I am getting as signal but the frequency doesn't add up. It's about 7MHz instead of the 9MHz expected from the Mixer. Back to the drawing board.
The switchable 9MHz Crystal Filter.
The output from the mixer is the sum and difference of the two input frequencies (5MHz and 4MHz). The crystal filter should attenuate all the frequencies but those very close to the 9MHz sum. Reading about others experience with these filters, and the suggestion to make the number of crystals switchable, I made the filter above. Each crystal added in series should narrow the bandwidth of the signal output. It will be mounted in an aluminum box to shield it from stray RF. Next, I'll see if this makes a difference in the output signal from the mixer.
Intermission...
This past spring, during pandemic lockdown, I started studying for the General Class amateur radio license - anything to keep busy and use the time for something constructive. I held a Novice (WV2LWW) and then a Technician Class license when I was in high school (WA2LWW). I let that license lapse during college, but took the Morse code and written theory test for the Technician Class license while teaching in the late 80's, some 20 years later. I had tried to test for the General license at that time but fell short of the ability to copy Morse code at 13 words per minute. As I remember it, the Technician and General Class theory tests were the same, with the difference being the Technician license only required that you copy code at 5 WPM as opposed the the 13 WPM required for the General. So I got the consolation prize of the Technician license.
In June, I took and passed both the Technician and General Class theory test. I applied to the FCC for the license call sign I had in the 80's and it was issued: N2IFV. The ability to read and send Morse code is no longer necessary to qualify for an Amateur Radio license. Not sure how I feel about that and evidently there is quite a bit of controversy within the ham radio community - at least with the older hams. I have started to re-learn the code but it is quite the challenge for me. The highest level I achieved once upon a time was 11 words per minute, but I would like to get up to the respectable 20 WPM to make it a useful method of communication. We'll see.
As of this writing, I haven't used the license. I guess I want to construct my own equipment. I'm not terribly interested in just "jawing" at people I don't know, but would rather have something to say about the equipment I am using, having built it myself. I still have some equipment from the 80's that seems to work, but would probably require replacing parts that tend to deteriorate over time to ensure that it would be reliable. Maybe some day.
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