Using LimeSDR Mini to align AM/shortwave radio

#1

Hi,

I’m still waiting on my LimeSDR Mini, but I recently picked up a 1950s transistor shortwave radio, and wanted to use the LimeSDR to generate the test signals into the IF mixer to align the radio. From a software perspective I’m comfortable with how to generate the signals, but I’m wondering about how to go about attaching the output of the Lime to the mixer of the receiver circuit without damaging the Lime.

I have a Ham It Up v1.3 which will do the downconversion to the 455kHz IF frequency of the receiver.

The alignment procedure calls for connecting an RF generator with a 400Hz tone AM modulated on a 455kHz carrier connected through a 0.05mfd capacitor to the mixer base which is exposed as a lug on the PCB.

Does anyone have any advice on hooking this up without blowing the TX side of the Lime?

#2

You could use a scope probe on 10X, allowing the resistance in the probe to isolate the Lime from the IF.

#3

Thanks for the tip!

I worry that the impedance of the scope probe is on the order of 1M ohm, which is a large mismatch from the 50ohm transmit path of the Lime. My limited understanding is that transmitting with no load or a highly mismatched load can cause reflections which damage the transmitter.

I wonder if I can just reduce the TX gain in software to keep reflected current to a safe level.

Connecting through a capacitor should block any DC coming back from the radio circuit into the Lime.

#4

From a software perspective, I put together some code in C with liquid-dsp and SoapySDR that’s modulating a 440Hz tone with AM in the 915MHz ISM band and transmitting on my bladeRF. Comes in loud and clear on an RTLSDR in CubicSDR, so I’m good to go.

My LimeSDR Mini still hasn’t been shipped from CrowdSupply yet as it seems they’re moving warehouses.

#5

High VSWR will hurt a 50W PA - the 10dBm PA in your Lime won’t even notice.