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]
