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Originally Posted by nemo_07[This is not good enough in either case, and primarily not because of loading ("de-Queing"), but because of significant detuning when touching a LC tank, be it IF or front end.
From the well known equation for a LC tank we can derive the value of relative detuning delta_F/F resulting from a small detuning capacitance Cin, being
(1) delta_F/F ~= -0.5*Cin/C, for Cin << C.
For example, with C = 100pF, Cin = 0.5pF we get -0.25% detuning, corresponding to -26.75 kHz at 10.7MHz IF.
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We are not aligning a narrow band FM IF stages. 0.25% detuning is nothing for wideband FM alignment. I wonder if you have ever tried aligned wither mono stereo IF strips for broadcast receivers before? I can easily do alignment for a FM mono radio "blinded" by the using output voltage method in synchronous tuning. However visual alignment is the king for both synchronous and staggered tuning.
I am not sure how you define C1...if C1 is the input capacitance of the probe including stray of the ground lead and clip. C1 acts like a voltage divider. The input capacitance for BF888 is 2.1pF. If C1 = 0.7, the effective input capacitance is 0.525pF.
The signal integrity on the scope and rise time is not my concern as I am probing sine waves at broadcast FM frequencies well below the resonance frequency of the LC network for the probe tip. Beside the resonance ringing can be suppressed by adding a simple Butterworth low pass filter at the output.
If you measure the S21 and S11 of the probe without the ground, the results are a total mess.
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Originally Posted by nemo_07Such probes are designed for looking at logic or other circuits with moderate or low impedances, not for probing Hi-Q LC tanks. Beside detuning, some of them will introduce measurable damping at 100MHz.
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How you do know? Have you tried? This guy probed the IF output for the residual output of FM front end tuners with multi-stage tuned bandpass filters. He used a Tektronix 500MHz P6202A with input capacitance of 2pF:
http://ham-radio.com/k6sti/swept.htm
I am so intrigued by his results that I bought a Tektronix P6202A on ebay USA last week. I am waiting for it to arrive. I have a Farnell triple output TOPS 3D power supply that will enable me to use it without the Tektronix power supply. I have attached the Rp and Xp charts for P6202A and P6201.
Correct me if I am wrong, Xp is the parallel equivalent reactance of the capacitive parts of the probe tip and FET input. Looking at the trend, it looks like the reactance of the total input capacitance dominating and the stray inductance is negligible. Rp is equivalent parallel resistance of the probe..I am not sure how it is calculated. For the P6201, the reactance for the stray inductance only starts to kick in above 800MHz.
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Originally Posted by G0HZU_JMR One thing to watch out for with any FET based follower circuit is (unwanted) negative resistance at the gate up at VHF. The BF998 has a high transconductance so will be very prone to producing negative resistance at the input if care is not taken with the choice of components at the source and gate pins.
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Funny you wrote that. Originally i used the same component values as in that Poorman's article and it didn't work. I changed the bias voltage with different R2 and R3 voltage divider , then it works all fine and dandy.
In the third attachment, I attached Z11 measured by Hirshbuscher's degree thesis using a 4-port VNA. I have no idea how to extract Z11 from S11 and phase angle. It is beyond my technical understanding. Basically, we attach a high impedance DUT to a 50 ohm transmission line, there is a massive mismatch in VSWRs between these two devices and hence high uncertainties in the Z11 measurements.