How do I get a stable measurement of Voltage and Frequency using an Oscilloscope on a VFO of a radio receiver?
The waveform is stable on the screen. I am only talking about the numerical voltage and frequency measurements.
The oscilloscope is an old HP 54600 100 MHz 2-channel digital scope from the 1990s.
The VFO is from the NorCal 40B CW Transceiver Kit from NM0S Electronics.
As part of the build of the transceiver for the 40 meter ham band (7 MHz), there is a need to measure the VFO output to make sure it is where I want it.
You take the VFO frequency plus frequency of a local crystal oscillator that is the tuned frequency of the receiver.
The measurement seems to bounce around 50 kHz from reading to reading every few seconds. So, is it the scope, the VFO, or is it me not measuring properly? So, I had to change to a HP 5314A frequency counter to measure the frequency - and the reading was stable. But not sure what to do about the voltage reading though.
The scope and 100 MHz scope probe are set to 10X and the scope probes have been compensated.
Same thing when I measure a crystal tester that is basically a Colpitts oscillator. The reading of the frequency and voltage bounces around each reading. I use the crystal tester to find the parameters of a crystal such as frequency so I could find a matched set of crystals in a bag of crystals. This is required for building filters that need multiple matched crystals.
Any suggestions?
Thank you,
Daniel
Some scope to the math, like measuring the frequency only with the data shown on the screen. In old one the software to calculate the frequency is sometimes poor. It can get a bit better when changing the horizontal scale to get more periods on the screen.
As I wrote in the 54600A thread the frequency measurement jumps, because that oscilloscope is not a counter and the frequency measurement there is just a tool showing aprox 4 digits frequency derived from the period measurement. You may get much more stable reading when you switch the Average ON (the 8/64/256 setting and you have to wait a little bit then), however.
The measurement of the frequency of a VFO (with the counter as well) has to be done either via a weak coupled inductance (a couple of turns) put 2-3cm off the inductor, or, in the case of a toroid inductor used in the VFO - coupled directly only at the VFO's buffered output (so after the second transistor - the buffer - coupled to the first VFO's oscillator transistor via a small, several pF large capacitor).
With coupling/loading the VFO with your probe directly on the LC or the transistor you will change the frequency of the oscillator (how much it depends on the actual frequency). While listening the beat in a radio you may hear the tone changes while messing around the VFO with the probe or your hands.
Building a "free running VFO" is a difficult exercise, such a VFO has to be properly shielded, mechanically perfectly stable (incl. the windings), its output buffered, and its LC has to be temperature compensated (even with 2-3MHz VFOs). Using the varicap diodes for tuning makes the situation even worse - either you TC the diode(s), or better use an air variable capacitor (mechanically stable one).
Better to go with crystal based PLL or DDS chips, like the popular Si5351, or AD9835/9851 etc for your aprox 2-3MHz VFO.
The freq reference in the many scopes I have seen is not great. So the resulting freq readout is not great also. Some scopes have separate freq ref to do the freq readout, most do not.Some scopes have a freq counter built in and use part of the input signal to trigger a freq counter, some use the trigger mechanism for the counter.
Apparently your scope uses the period measurement which can be different with small phase shifts and can change rapidly from cycle to cycle. The accuracy of all the scope counters I have seen is poor, or more that the stability of the reading is poor, even my Digital Agilent Infinium Scope bounces around. Fortunately there is an Average measurement.
I don;t know if your scope has a facility for an external freq reference, sometimes this helps quite a bit.iMo knows a lot more about this particular scope than I do, try his suggestion about the time average.
Also freq counters count the number of cycles in a time interval. so inside that time interval there may be slightly different freq and the counter gives the total count or average freq. Again this is only as good as the freq reference of the counter.
However I would trust the counter rather than the scope as far as freq stability.
The ultimate test will be assembling the kit and hearing it on a receiver.
Also counters load the output of what they are measuring more than a scope does usually. Maybe loading the circuit stabilizes it.
Thank you everyone for your replies.
My scope has a more stable measurement for Voltage and Frequency now based on your suggestions.
See attached photos.
Video is my NorCal 40B CW Transceiver is receiving...
I am working on the transmitter blocks now...
iMo,
What toroid can I use? Maybe a FT37-43? I have many in my junk box. Otherwise, I can just go to Toroid King at kitsandparts.com and get some.
I also saw a ham stick a capacitor at the end of his probe to check the VFO.
As I mentioned, the VFO is part of a radio kit - the NorCal 40B CW Transceiver Kit
https://sites.google.com/nm0s.com/home/norcal-40b?authuser=0So, I cannot just switch that out. Later, I want to build the radio again using the book Electronics of Radio by David Rutledge. This books also has you do various experiments using various test equipment like an Oscilloscope.
Also keep in mind, I also measured a G3UUR Crystal Tester that is basically a Colpitts Oscillator. If you have a book like Experimental Methods in RF Design, it talks about this crystal tester.
Link is for a ham who homebrewed this crystal tester.
https://kk9jef.wordpress.com/2016/04/25/g3uur-crystal-oscillator/And I have built a VFO already for a Direct Conversion Receiver that uses a Si5351A clock synthesizer, Arduino Nano clone, i2c LCD Display, and a rotary encoder. Wrote my sketch. And it works.
I think iMo was writing about ways of getting a reading from an oscillating circuit without disturbing the oscillator freq very much.
If you put a loop lead near the oscillator, you will get a signal. It will be a small signal but doing this will not change the frequency of the osc much. A loop would be attaching the tip of your scope probe to the ground lead of your scope and putting it hear the oscillating circuit.This small antenna picks up the signal. A few loops of this antenna gives a stronger signal. The signal would be very small and maybe not visible on a scope. I use a Spectrum Analyzer for this and it can work quite well. If the inductor is a Toroid, it is harder to pick up the signal since the signal is contained mostly in the Toroid. Other inductors have lines of magnetic force outside the coil.
You can use a scope probe on a SA easily. The old time instrument, the Grid Dip Meter, also did something like this, coupling to the magnetic field of the coil. You can also just touch the probe to a Xtal case and you will get a signal.Adding a cap to the scope input still will change the impedance and hence the freq in the oscillator somewhat. If you do this after the buffer, you should not change the freq being monitored.
As iMo suggested. there is usually a buffer amplifier transistor after the oscillator to bring up the signal. This is the place to monitor the VFO frequency. Here is where you would put in a maybe 100 pF cap on the buffer Output and monitor the freq from this cap. If you wish at this point in the circuit you can put in an amplifier for monitoring the frequency on a permanent basis, for only 2-3 MHz you can use an op amp with high impedance inputs, and then you will have a nice output to monitor your oscillator. Use maybe a 100 pF cap to the input of the op amp from the output of the buffer amplifier. As I recall some older Ham radios had this feature, especially for the IF. Then a specialized counter would add or subtract another freq to get the actual transmit freq readout.