Is my oscilloscope affecting my tuning circuit?

Question

I am building an AM Radio and I have constructed a tuner circuit. See below.

^{simulate this circuit – Schematic created using CircuitLab}

However, I cannot use this circuit, because even though in my calculations it works, I am not able to see the signal for the 570kHz band when it's connected to the oscilloscope. The 570kHz band will only show up if I drop my inductance down to about 250 uH. To me, this doesn't make sense and so I'm trying to track down the culprit.

After finding the band 570kHz, I measured the capcitor and found that my capacitor was set to about 234 pF.

Thinking this was odd, I put my LCR meter to the inputs of the oscilloscope and found that it has an inductance that fluctuates from 1000mH to 2000mH.

Is my oscilloscope affecting my tuning circuit, or is there parasitic capacitance somewhere in my circuit that is affecting it?

I want to be able to use the circuit I've described here so I can get a better band pass filter (I want high L and low C).

Details: - the oscilloscope I'm using is an Iwatsu SS-5702 oscilloscope -- quite an old model - my inductor is hand made and measure to be 1960uH - my antenna is just a piece of wire - I'm using a spring board to connect the parts

Answer 1

Most old scope probes were in the 20 to 30pF range which will load your tank circuit to a lower frequency by the range of tuning cap shown in your schematic.

Perhaps your LCR meter is reading higher values at 120 Hz or 1kHz than what might be expected at 650kHz, but that is a separate concern.

The capacitance of the Probe cable is in parallel with the input for the scope front panel. A 10:1 R probe reduces load current by 1/10th using a 9M series R to get 1/(1+9). The 10:1 probe also reduces the input capacitance of added coax capacitance 20~30pf/ft and Scope front panel, which may be 15 to 30pF for that vintage.

Being curious I looked up the typical values for IWATSU SS-0060 series probes for the SS-5702

Scope input: 1 Meg//30 pF +/-3 10:1 probe :10 Meg//23 pF (adj)

They do make FET buffered scope probes, but are rather expensive and prone to ESD failures.

To test the circuit you would need a low capacitance, high impedance transistor buffer, which your circuit will need any ways, perhaps with gain.

You can use a CMOS VCO to sweep it with a wire tray capacitance couple to your antenna. Often the tank circuit is put on the collector of a common emitter circuit , high gain, low noise or with a low noise FET front end pre-amp then high gain with more filtering and IF mixer.

A useful visual aid is called an RLC Impedance Nomograph, that shows the impedance of all RLC parts as they intersect on the Z vs f chart.

For example 30pF @500kHz is approximately 10kOhm which means your tank circuit gain with 1Meg load, could be 1M/10k=100=Q. Obtaining more Q gain makes the circuit very sensitive to temperature drift of copper coil and capacitor, and also reduces the bandwidth by the same ratio compared to centre f.