How to Verify 70 MHz Output From Unbranded VCO With Spectrum Analyzer and DIY Antenna

147 21

Best answers

How can I tell whether my unbranded 70 MHz VCO is actually transmitting when my spectrum analyzer only shows a small change in the noise floor?

A small change in the noise floor does not prove the VCO is transmitting; if it is oscillating you should see a clear narrow peak at one frequency [#21684809] Make sure the spectrum analyzer has its own antenna, place it close to the VCO antenna, and use shielding such as aluminum foil if needed to reduce interference from nearby signals like a broadcast station [#21684809] [#21684816] If the control voltage is left undefined, the output frequency will also be undefined, so apply a known control voltage; one suggestion is to start around half the supply rail and use a potentiometer from ground to the rail [#21684812] [#21684825] A stable oscillator and analyzer should show the peak in the same place each time, so moving spikes usually mean the settings, sweep speed, bandwidth, or control-voltage range need checking [#21684825] Do not add a 30 dB amplifier just to search for the signal, because a claimed 10 dBm output into 50 ohms is already about 2 Vp-p and extra gain can clip the waveform or just raise the noise floor [#21684819] [#21684822]

Generated by the language model.

Report a violation of the law

Reply | New topic

#1 21684805 25 Jul 2020 00:25

david lopez david lopez

Anonymous

Post #1
21684805 25 Jul 2020 00:25

I bought a voltage controlled oscillator from a Chinese seller. I don't think the seller listed the model of voltage controlled oscillator. I don't think I can find the datasheet. The seller says the output is 10 dBm. The seller says if a red light turns on. The voltage controlled oscillator has turned on and is connected to the right supply voltage. The upper pin is connected to the positive terminal. The pin below that is connected to the negative terminal. 2 pins are not connected to anything. The seller says if the control voltage is 0 volt, the output is 70 megahertz. I connected the voltage controlled oscillator to a coaxial cable. I shaved off the insulation and I shaved off the outer copper layer. I removed the outer copper layer. The coaxial cable is suppose to act as an antenna. The antenna radiates radio waves. I have a spectrum analyser. The spectrum analyser is suppose to pick up radio waves. The blue lines on the screen of the spectrum analyser are at roughly -25 dB. When I connect the 12.28 volt battery to the voltage controlled oscillator the blue lines on the screen are reduced to roughly -30. If the battery is not connected to voltage controlled oscillator I see this Does this mean there is a radio signal.
ADVERTISEMENT
#2 21684806 25 Jul 2020 02:34

Elizabeth Simon Elizabeth Simon

Anonymous

Post #2
21684806 25 Jul 2020 02:34

I don't see the picture you are referring to.
ADVERTISEMENT
#3 21684807 25 Jul 2020 02:58

david lopez david lopez

Anonymous

Post #3
21684807 25 Jul 2020 02:58

Why not. When I look at the screen on the spectrum analyser, the blue lines have a certain height. This height reaches -25 dB. When i connect the battery the height reaches roughly -30 dB. The vertical axis is intensity. At regular intervals it marked -60 dB, -55 dB, -50 dB, -45 dB, -30 dB, etc. The horizontal axis is frequency. I want this question answered. Does this mean the antenna is transmitting a signal?
#4 21684808 25 Jul 2020 02:59

david lopez david lopez

Anonymous

Post #4
21684808 25 Jul 2020 02:59

Why not. When I look at the screen on the spectrum analyser, the blue lines have a certain height. This height reaches -25 dB. When i connect the battery the height reaches roughly -30 dB. The vertical axis is intensity. At regular intervals it marked -60 dB, -55 dB, -50 dB, -45 dB, -30 dB, etc. The horizontal axis is frequency. I want this question answered. Does this mean the antenna is transmitting a signal?
ADVERTISEMENT
#5 21684809 25 Jul 2020 06:10

Elizabeth Simon Elizabeth Simon

Anonymous

Post #5
21684809 25 Jul 2020 06:10

What I would expect to see on the spectrum analyzer is a peak at one frequency if your oscillator is oscillating. It sounds like all you are seeing is the noise. How is your spectrum analyzer set to receive the signal? You need to have an antenna on the spectrum analyzer to receive the RF. One similar to what you have on the oscillator should work well enough.One way to check that you can receive RF is to see if you can see a peak at the frequency of a local broadcast station.
#6 21684810 25 Jul 2020 06:39

david lopez david lopez

Anonymous

Post #6
21684810 25 Jul 2020 06:39

I already did that. I see a peak at 100 megahertz. I listen to a local radio station at that frequency. I am sure the spectrum analyser is working.
#7 21684811 25 Jul 2020 06:44

david lopez david lopez

Anonymous

Post #7
21684811 25 Jul 2020 06:44

I think the bottom pin and the pin above that are to receive the control voltage. They are not connected to anything. Then the voltage is undefined. If the voltage is undefined, will the voltage controlled oscillator generate radio waves at any frequency?
#8 21684812 27 Jul 2020 21:36

Elizabeth Simon Elizabeth Simon

Anonymous

Post #8
21684812 27 Jul 2020 21:36

If the control voltage is undefined then the oscillator will generate an undefined frequency. You need to put a voltage in to define the output frequency. You may need to put the antenna of the spectrum analyzer next to the antenna of the oscillator. Also if the frequency is near 100 Megahertz you may not see it because of the radio station on that frequency.
#9 21684813 28 Jul 2020 00:25

david lopez david lopez

Anonymous

Post #9
21684813 28 Jul 2020 00:25

so if the voltage is undefined, then the frequency is undefined, but the voltage controlled oscillator will still transmit a radio wave?
#10 21684814 28 Jul 2020 03:53

Elizabeth Simon Elizabeth Simon

Anonymous

Post #10
21684814 28 Jul 2020 03:53

Yes it should. Provided that it is working at all...But it may be that the output is very low so that you have trouble seeing it if there is another signal nearby. Do you have an oscilloscope that you can use to look at the oscillator output to see that it is working?
#11 21684815 28 Jul 2020 04:18

david lopez david lopez

Anonymous

Post #11
21684815 28 Jul 2020 04:18

no, i live below the poverty line, an oscilloscope would be too expensive for me.
#12 21684816 31 Jul 2020 06:44

Elizabeth Simon Elizabeth Simon

Anonymous

Post #12
21684816 31 Jul 2020 06:44

Not having an oscilloscope makes troubleshooting this type of thing more difficult.One possible way you could do it is to have the antenna for the output next to the antenna for the spectrum analyzer then add a shield to cover them. the idea is to keep the radio station and similar signal out. You will need to ground the shield to the ground on the spectrum analyzer.This can be done with aluminum foil or similar. Just take care that you don't short your circuit to the foil.
ADVERTISEMENT
#13 21684817 31 Jul 2020 07:37

david lopez david lopez

Anonymous

Post #13
21684817 31 Jul 2020 07:37

i am thinking of buying an amplifier on the internet. i have seen ones rate at 30 dB. i think that is a 1000 fold increase. that will i think increase the signal 1000 fold. will that solve this problem?
#14 21684818 31 Jul 2020 07:39

david lopez david lopez

Anonymous

Post #14
21684818 31 Jul 2020 07:39

i am thinking of buying an amplifier on the internet. i have seen ones rate at 30 dB. i think that is a 1000 fold increase. that will i think increase the signal 1000 fold. will that solve this problem?
#15 21684819 01 Aug 2020 02:02

Richard Gabric Richard Gabric

Anonymous

Post #15
21684819 01 Aug 2020 02:02

If the claimed output of the oscillator is 10dBm, that is 2Vp-p , I am assuming a 50 Ohm system, which is implied by the dBm. Therfore there is no point in using an amplifier, any further amplification on top of what the oscillator should provide is only going to cause at best severe clipping, with an increase in harmonic output. If the oscillator is not working, then gain will just increase the noise floor. I really think that you need to try harder to get some sort of data on the oscillator, otherwise, as Elizabeth said, you will probably need to somehow get your hands, even temporarily, on an oscilloscope.Cheers,Richard
#16 21684820 01 Aug 2020 06:32

david lopez david lopez

Anonymous

Post #16
21684820 01 Aug 2020 06:32

you mentioned severe clipping. could you explain what that is? and provide a picture? does that mean the signal increases, then remains at the same level for a period of time, then decreases, and then repeats this event on and on? instead of increasing up to a point, then decreasing immediately.
#17 21684821 01 Aug 2020 06:33

david lopez david lopez

Anonymous

Post #17
21684821 01 Aug 2020 06:33

you mentioned severe clipping. could you explain what that is? and provide a picture? does that mean the signal increases, then remains at the same level for a period of time, then decreases, and then repeats this event on and on? instead of increasing up to a point, then decreasing immediately.
#18 21684822 01 Aug 2020 08:03

Richard Gabric Richard Gabric

Anonymous

Post #18
21684822 01 Aug 2020 08:03

Imagine that your 2V p-p signal is now 2000V p-p (gain of 1000). Now the supply rail on the amplifier is say 10V, so your signal will now transition from ground to 10V very rapidly (It will look like a square wave in fact, since the large part of the peak of the wave form is at either ground or the supply rail). In reality such severe over drive will cause all sorts of non linerity problems, so there may well be other issues as well, assymertry in the wave form is likely (the duty cycle will change since the amplifier is likely to be AC coupled). Additionally, the spectral purity is degraded, as you approach a square wave the odd order harmonics increase, and assymetry will increase the harmonic content further depending on the duty cycle.If the oscillator noise is say 1mV, then with a gain of a 1000 you can expect a volt of noise.
Cheers,Richard
#19 21684823 01 Aug 2020 08:13

david lopez david lopez

Anonymous

Post #19
21684823 01 Aug 2020 08:13

i have tried connecting the control voltage pins to a 4 volt lithium ion cell. i think i saw spikes appear on the screen. i don't think the spikes appear at the same spot each time. does this mean the voltage controlled oscillator is producing a radio signal?
#20 21684824 01 Aug 2020 08:14

david lopez david lopez

Anonymous

Post #20
21684824 01 Aug 2020 08:14

i have tried connecting the control voltage pins to a 4 volt lithium ion cell. i think i saw spikes appear on the screen. i don't think the spikes appear at the same spot each time. does this mean the voltage controlled oscillator is producing a radio signal?
#21 21684825 01 Aug 2020 14:09

Richard Gabric Richard Gabric

Anonymous

Post #21
21684825 01 Aug 2020 14:09

Dont't think!!, they either do or they don't, a stable oscillator and a stable spectrum analyser would definitely present them in the same location. What are the specs on your spectrum analyser and what settings are you using? Sweep speed and band width have to be set correctly (normally they interlock in some fashion) or it is possible to completely miss what you are looking for, since the oscillator output will be a very narrow band signal. Additionally, if you don't know what the limits are on the control voltage, you may be running it out of range into a region of instability. I suggest that you try a voltage around half the supply rail for starters. The control voltage is possibly operating into varactor diodes, for example, which will tolerate a high impedance from the control source, so I suggest you try a potentiometer that will take the control voltage over a range somewhere between ground and the supply rail, and start at a middle setting.Cheers,Richard
#22 21684826 01 Aug 2020 15:46

david lopez david lopez

Anonymous

Post #22
21684826 01 Aug 2020 15:46

i will check.
Create an account, log in here. You will receive points by participating in discussions.
Join this discussion.

Install Elektroda application

Reply | New topic

Report a violation of the law

Topic summary

✨ A user purchased an unbranded voltage controlled oscillator (VCO) from a Chinese seller claiming a 70 MHz output at 0 V control voltage and 10 dBm output power. The user connected the VCO to a coaxial cable stripped to act as a DIY antenna and attempted to detect the signal using a spectrum analyzer. The spectrum analyzer showed peaks around -25 to -30 dB, but the user was uncertain if these represented the VCO output or noise. The user confirmed the spectrum analyzer's functionality by detecting a local broadcast at 100 MHz. It was advised that the VCO requires a defined control voltage to produce a stable frequency output; leaving control voltage pins floating results in an undefined frequency. Suggestions included placing the VCO antenna close to the spectrum analyzer antenna and shielding both to reduce interference from local radio stations. The user lacks an oscilloscope, complicating direct waveform verification. Amplification of the signal was considered but cautioned against due to potential severe clipping, waveform distortion, and increased harmonic content if the VCO output is already at 10 dBm (2 V peak-to-peak in a 50 Ω system). Proper spectrum analyzer settings (sweep speed, bandwidth) and controlled variation of the control voltage, ideally via a potentiometer from 0 V to supply voltage, were recommended to observe stable frequency peaks and verify VCO operation.
Generated by the language model.

FAQ

TL;DR: A 10 dBm VCO should produce about 2 Vp‑p into 50 Ω; avoid over‑amplifying because “any further amplification…causes severe clipping.” [Elektroda, Anonymous, post #21684819]

Why it matters:** This FAQ helps budget‑limited builders verify a ~70 MHz VCO with a spectrum analyzer and DIY antennas, without a datasheet or oscilloscope.

Quick Facts

Claimed VCO output: 10 dBm ≈ ~2 Vp‑p into 50 Ω; extra gain risks distortion. [Elektroda, Anonymous, post #21684819]
Floating control voltage = undefined frequency; apply a defined CV to tune output. [Elektroda, Anonymous, post #21684812]
Fast check that your analyzer/antenna work: look for a local FM peak near 100 MHz. [Elektroda, Anonymous, post #21684810]
To reduce broadcast bleed‑through, couple antennas closely and add grounded foil shielding. [Elektroda, Anonymous, post #21684816]
Wrong sweep/RBW settings can hide a narrow VCO peak entirely. [Elektroda, Anonymous, post #21684825]

How do I verify my unbranded VCO around 70 MHz on a spectrum analyzer?

Attach a simple antenna to the analyzer, place it near your VCO’s coax “antenna,” and scan around the expected frequency. First confirm the analyzer sees a known local station, then look for a distinct narrow peak from your VCO. Keep the antennas close for better coupling. [Elektroda, Anonymous, post #21684809]

Why did my noise floor drop from about −25 dB to −30 dB when I power the VCO?

That change alone does not prove transmission. A working VCO appears as a clear, narrow peak at a specific frequency. Ensure the analyzer has an antenna and proper span, then check near 70 MHz while the VCO is powered. [Elektroda, Anonymous, post #21684809]

What happens if the control voltage pin is left floating?

A floating control voltage makes the output frequency undefined. Apply a known control voltage to set the frequency, then observe the analyzer for a stable peak. [Elektroda, Anonymous, post #21684812]

How should I apply control voltage without a datasheet?

Use a potentiometer to sweep the control pin from ground to the VCO supply, starting near mid‑supply. This keeps tuning within a reasonable range and avoids unstable regions. “Start at a middle setting.” [Elektroda, Anonymous, post #21684825]

Can a 30 dB RF amplifier help me see the signal?

If your VCO truly outputs 10 dBm, adding high gain often drives amplifiers into clipping and increases harmonics. If the VCO isn’t oscillating, an amplifier only raises the noise. Verify oscillation before adding gain. [Elektroda, Anonymous, post #21684819]

What exactly is clipping here?

Clipping is when an overdriven amplifier forces the waveform to the supply rails, turning a sine into a squarer wave and polluting the spectrum. “As you approach a square wave the odd harmonics increase.” [Elektroda, Anonymous, post #21684822]

I see spikes that move when I connect ~4 V to the control pin—does that prove RF output?

Not yet. A stable oscillator and analyzer should show peaks in the same place. Set sensible sweep speed/RBW and tune the control voltage slowly within range. If peaks drift, your settings or control range may be off. [Elektroda, Anonymous, post #21684825]

How do I keep strong broadcast stations from masking my VCO?

Place the VCO and analyzer antennas very close, then add a grounded aluminum‑foil shield around both to block outside signals. Avoid shorting your circuit to the foil. [Elektroda, Anonymous, post #21684816]

How can I sanity‑check my analyzer setup quickly?

Tune the analyzer to a known FM broadcast around 100 MHz and verify a strong peak. This validates your antenna and analyzer chain before chasing your VCO. [Elektroda, Anonymous, post #21684810]

What antenna should I use at the analyzer input?

Use a small improvised antenna similar to your VCO’s radiator (e.g., short coax stub). Proximity coupling at tens of centimeters or less works for bench tests. [Elektroda, Anonymous, post #21684809]

What does 10 dBm mean in volts for a 50 Ω system?

10 dBm is commonly stated for 50 Ω loads and corresponds to about 2 Vp‑p at the output in this discussion. Treat it as a strong bench‑level signal. [Elektroda, Anonymous, post #21684819]

I don’t own an oscilloscope—what’s a simple three‑step method to verify output?

Put a short antenna on the analyzer and another on the VCO, tips a few centimeters apart.
Wrap both with grounded foil to reject outside stations.
Sweep the analyzer near 70–110 MHz while adjusting the control voltage slowly. [Elektroda, Anonymous, post #21684816]

What is a VCO in plain terms?

A Voltage‑Controlled Oscillator generates RF whose frequency changes with an applied control voltage; leave it floating and the frequency wanders. [Elektroda, Anonymous, post #21684812]

Which analyzer settings matter most so I don’t miss the carrier?

Use a narrow resolution bandwidth and appropriate sweep speed. If RBW is too wide or sweeps are too fast, a narrow carrier can vanish in the display. [Elektroda, Anonymous, post #21684825]

Generated by the language model.