04-08-2022, 12:16 PM
Great experimenting!
Don't chuck out the apparently dead crystal! Try it in a few transistor and IC crystal circuits. If it's an overtone crystal it may oscillate at fundamental on an unbuffered IC inverter.
I'm sure Trevor knows, but overtones are higher frequency mechanical modes, so are not integer multipliers of the harmonics. A fundamental mode osc needs no tuned parts, but an overtone mode needs additional L & C in the feedback to select the required overtone (a 3rd Overtone crystal can sometimes be persuaded to oscillate at 5th. Some 40 MHz approx crystals are fundamental and others are 3rd overtone, so won't always work in same circuit. Harmonics are different and always an even or odd multiple of the oscillation, so need a separate oscillator followed by an amp tuned to desired harmonic. Frequency doubling is simple, but 4th and 6th are best done by two doubler and doubler and tripler. The 3rd and 5th are easy directly, 7th needs more care.
Above 40 to 50 MHz the crystals (such as 101 MHz) are almost always overtone mode and in a simple IC osc + counter crystal checker will either run at the fundamental (no integer relationship) or not at all.
Some crystal circuits will work with some resonators or even an IF Filter (455 KHz, 6.0 , 6.5, 10.7, etc). Higher frequency monolithic filters ( 20 MHz to 200 MHz) may be SAW types and need more gain. A narrow filter is more stable than a wider one as an oscillator.
My last modulator/mixer - oscillator experiments were with the double balanced mixer-osc SA612A IC with on board regulator for osc. Oscillators benefit from a noise free stable supply, as the supply noise and ripple can AM, PM and FM the oscillator. This matters for a LO on TX or RX on very poor signal links, not a local RF source.
Anyway, by default the SA612 has a DSBSC output, nothing on the output from the LO or input (BB or LO). It can do PM by adding in the LO from pin 7 to the output via a buffer at the correct phase. Then multiply for NBFM, or more for WBFM.
The trick to get a carrier for AM is to deliberately unbalance the input pins at DC, by pulling one down more with a resistor to 0V and pulling the other up a little to noise free regulated voltage via a resistor, though just one pull down will work. The input will ALSO appear at the output, but if it's baseband that's not an issue. In normal use the output is Fosc-Fin, Fin- Fosc and Fosc + Fin, filtering giving the desired signal. With 38 kHz and L to one input and R to other input you'll get something like the stero subcarrier, then divide the 38 KHz by 2 and add it (19kHz) in to the L+R and 38 kHz DSBSC. Except it's pointless as an entire IC single chip 76 MHz to 108 MHz FM Stereo modulator ready made is about €8.
Anyway, with adding one or two resistors at pin 1 & 2 (input one to 0V and one maybe to LED on lamp) it works as AM for 60KHz to 500 MHz and audio or video baseband. Positive or Negative modulation.
Vcc 4.5V to 8.0V
Current about 2.6 mA
Input DC to 500 MHz
LO some 10s of kHz to 200 MHz or more.
Can be LC, crystal or ceramic resonator. A varicap can be added for a PLL (add external dividers & comparators) but not worthwhile with excellent LMX series now. or direct FM.
It's actually easy to get LC to work, but it's tricky to get crystals and resonators to work. Overtone mode crystals need a separate oscillator driving pin 6, which is disappointing, or else I'm doing it wrong!
Don't chuck out the apparently dead crystal! Try it in a few transistor and IC crystal circuits. If it's an overtone crystal it may oscillate at fundamental on an unbuffered IC inverter.
I'm sure Trevor knows, but overtones are higher frequency mechanical modes, so are not integer multipliers of the harmonics. A fundamental mode osc needs no tuned parts, but an overtone mode needs additional L & C in the feedback to select the required overtone (a 3rd Overtone crystal can sometimes be persuaded to oscillate at 5th. Some 40 MHz approx crystals are fundamental and others are 3rd overtone, so won't always work in same circuit. Harmonics are different and always an even or odd multiple of the oscillation, so need a separate oscillator followed by an amp tuned to desired harmonic. Frequency doubling is simple, but 4th and 6th are best done by two doubler and doubler and tripler. The 3rd and 5th are easy directly, 7th needs more care.
Above 40 to 50 MHz the crystals (such as 101 MHz) are almost always overtone mode and in a simple IC osc + counter crystal checker will either run at the fundamental (no integer relationship) or not at all.
Some crystal circuits will work with some resonators or even an IF Filter (455 KHz, 6.0 , 6.5, 10.7, etc). Higher frequency monolithic filters ( 20 MHz to 200 MHz) may be SAW types and need more gain. A narrow filter is more stable than a wider one as an oscillator.
My last modulator/mixer - oscillator experiments were with the double balanced mixer-osc SA612A IC with on board regulator for osc. Oscillators benefit from a noise free stable supply, as the supply noise and ripple can AM, PM and FM the oscillator. This matters for a LO on TX or RX on very poor signal links, not a local RF source.
Anyway, by default the SA612 has a DSBSC output, nothing on the output from the LO or input (BB or LO). It can do PM by adding in the LO from pin 7 to the output via a buffer at the correct phase. Then multiply for NBFM, or more for WBFM.
The trick to get a carrier for AM is to deliberately unbalance the input pins at DC, by pulling one down more with a resistor to 0V and pulling the other up a little to noise free regulated voltage via a resistor, though just one pull down will work. The input will ALSO appear at the output, but if it's baseband that's not an issue. In normal use the output is Fosc-Fin, Fin- Fosc and Fosc + Fin, filtering giving the desired signal. With 38 kHz and L to one input and R to other input you'll get something like the stero subcarrier, then divide the 38 KHz by 2 and add it (19kHz) in to the L+R and 38 kHz DSBSC. Except it's pointless as an entire IC single chip 76 MHz to 108 MHz FM Stereo modulator ready made is about €8.
Anyway, with adding one or two resistors at pin 1 & 2 (input one to 0V and one maybe to LED on lamp) it works as AM for 60KHz to 500 MHz and audio or video baseband. Positive or Negative modulation.
Vcc 4.5V to 8.0V
Current about 2.6 mA
Input DC to 500 MHz
LO some 10s of kHz to 200 MHz or more.
Can be LC, crystal or ceramic resonator. A varicap can be added for a PLL (add external dividers & comparators) but not worthwhile with excellent LMX series now. or direct FM.
It's actually easy to get LC to work, but it's tricky to get crystals and resonators to work. Overtone mode crystals need a separate oscillator driving pin 6, which is disappointing, or else I'm doing it wrong!
