I completed a few days ago the VFO for my BitX40. The receiver is quite sensitive, I can hear easily a 7MHz 1uV unmodulated RF signal applied to the RF port.
For transistor Q1 I used a BC547B. I will replace it later with a lower noise transistor.
Ideally, I should compare the lowest levels applied that can generate an audio output with a SINAD = 15 and see the improvement brought by a lower noise transistor.
Today I plotted the frequency response of the RF and IF Path together.
Because my spectrum analyser + tracking generator does not allow me to enter a frequency offset between generator and analyser,
I had to use RF signal from an external generator and then build the graph point by point, using trace on maximum hold.
I used a level of -60dBm applied to the Rx Antenna Input and I connected the spectrum analyser at the output of the 4dB PI attenuator made with 2 x 220 ohm and 22 ohm resistors.
I removed the 100 ohm pot from the board and connected the input of the spectrum analyser to the cursor point.
Please note that the 7MHz input bandpass filter is not built onboard at this stage and I have not used any filter during these tests.
The peak response has a level of -15dBm, which results in an overall gain of 45dB (before the last mixer and without input filter).
For my 10 MHz crystal filter, the flatness in the band is better than 2dB and the 3dB bandwidth is 2.54kHz.
Using 100 pF around the crystal filter is a good method for matching the impedance of the filter to surrounding circuits (initially I was skeptical about this method).
In order to perform these measurements while hiding the signal from BFO, I adjusted the BFO frequency to the left of the crystal response (there is a bit of spurious signal appearing in the photos).
Normally, the BFO signal for the band of 7 MHz should appear near the right shoulder of the crystal filter.
Here is the RF Spectrum of the Crystal Filter. I selected for the closest matches among 20 pieces.