Measuring frequency of LC circuit (around 100MHz) with frequency meter

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Bobes

Joined Mar 19, 2020
44
Hi!
I bought this frequency meter https://www.ebay.com/itm/1-MHz-1-1GHz-LED-Frequency-Counter-Tester-Measurement-Board-For-Ham-Radio/113260617263?epid=1531936276&hash=item1a5edbde2f:g:lwQAAOSwtLtbo68a about 2 weeks ago and I still cannot figure out, how to measure LC circuits with it. I built one transistor oscillator (FM transmitter with 1 transistor) and connected meter parallel to the LC circuit. I expected frequency of around 100MHz, but it displayed only about 6MHz. Why am I asking this? I need to measure frequencies in homemade tube radio (fremodyne) and I need accurate enough (+-5%) measuring instrument. Maybe, the better way would be to make inductance meter for homemade coils, but I cannot find any simple enough circuit on the Internet - I haven't PIC or LM339 as on many schematics. Therefore, I wanted to ask for a help someone more experienced, for these questions: Is it possible to measure frequencies with this frequency meter? And if you had some simple schematic of inductance meter (10s or 100s nH), can you post it here? I tried this one https://makingcircuits.com/blog/1-5-v-inductance-meter-circuit/ but I don't think it's accurate. Any help is appreciated. Thank you for your answers!
 

Papabravo

Joined Feb 24, 2006
13,456
There could be several reasons for the observed results in no particular order of significance:
  1. The instrument is loading your oscillator and pulling the frequency
  2. The instrument is reducing the level down into the noise
  3. The oscillator is oscillating at the wrong frequency
  4. You are not measuring what you think you are measuring
Possible solutions include:
  1. Buffering the output so it looks like 50 Ω impedance at 100 MHz.
  2. Building a 1 stage RF amplifier with a 50 Ω output to connect to the instrument to see if the oscillator can actually transmit
  3. Double check your results against a known good instrument
  4. Double check your instrument against a known good source from an RF signal generator
A schematic diagram of your test setup and one stage oscillator might be helpful
 
Last edited:

Janis59

Joined Aug 21, 2017
1,143
One fine method is to solder the Clapp oscillator and measure the frequency at Source resistor. More exact method is to apply the DIP-meter and measure the exact frequency of DIP. Third is apply the Zener diode based white noise generator circuit, where adding the parallel tank may provide the measurements in the output, the buffer is welcome before meter.
 
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