Measuring the in application performance of the BPF of the tinySA #tinysa
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When you make the BPF and measure it using your nanoVNA you hope the BPF will perform as measured, but are you sure it does?
Now it is possible to use a buildin function of the tinySA to measure the performance of the BPF after it is integrated You have to set the Sweep to "First IF" instead of "Regular" and set the first IF 10MHz lower then its normal value. When sweeping with these settings the RX SI4432 will track the LO SI4432 so together they act as a scalar network analyzer. This is possible as the mixer is not perfect so sufficient signal will leak from the LO port to the RF port. As we can not sweep negative? frequencies we have to offset the first IF to a lower frequency to make sure we sweep the whole passband and the outside the passband. The result is a bit disappointing. The passband of the BPF is correctly centered at 10MHz because we set the first IF 10Mhz lower. The width is also correct, about 1MHz. The first problem is the very high leakage between the LO SI4432 and the RX SI4432 that create a -50dBm background obscuring the out of band rejection below -50dBm. During normal measurement this is not a problem as normally the RX stays fixed on the first IF frequency. Any form of shielding will also reduce this but it shows how easily signals at 433MHz are leaking. The coupling is caused by the capacitor between the last SAW filter and the SMA connector together acting as a antenna for the signal radiating from the LO SI4432. Completely disconnecting the BPF drops the leakage to -55dBm. Removing the SMA female-female connector from the RX SI4432 drops the leakage to -70dBm. The second problem is the bumpy passband.This will generate changes in sensitivity when doing spur rejection. Some more tuning to do. -- HBTE Files section:?/g/HBTE/files Erik, PD0EK |
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Erik,
MANY THANKS for the .cfg file - that really helps us all..! Is there a certain configuration that the TinySA needs to be in for this measurement? I'm thinking of the IF input on the mixer being (I think) un-terminated and may have influence on the measurement? Do we need to configure the TinySA into a different RF configuration to make this measurement? Please let us know so we're following your work appropriately. If there's no changes needed from the configuration we all use to make spectrum measurements now then please let us know that, too. Again, many thanks for your efforts on this Erik -you cannot know the avalanche of people following this project and other Hams that want to get in and try this, too...it's remarkable... 73 de Marty KN0CK |
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Erik,
Thanks much for the further guidance - - Once I rebuilt by filter and tested it, here's my results as attached. Looks GREAT..! BTW - My BPF assembly does not use any capacitors or coils - I built mine with Qualcomm 360kHz BPFs, two micro-coax-SMA cable assemblies and I took a chunk of the micro-coax and connected it between both filters and it just gave me the most wonderful results - the pictures really show it. 73 de Marty, KN0CK |
Marty Wittrock
