LT5517 radio receiver


I’m beginning to try getting into some RF stuff.

I decided to start with a SDR receiver, based on the LT5517 chip.
You can find it’s data sheet here: LT5517

The chip is a quadrature demodulator that it’s specified from 40MHz to 900MHz. It can be pushed to lower frequencies by increasing the LO drive.
It works pretty good down to 3.5MHz ( 7MHz LO ) with ~ 0dBm drive.

Once LO gets over 40MHz ( 20MHz RX ) the LO can be as low as -15dBm with no problems. Usually I keep it at -10dBm for convenience – the signal generator I’m using now to generate the LO has a dedicated key to modify the output signal in 10dBm steps.

The mixer’s output bandwidth it huge – 130MHz and this generate some problems: if the input filters are not good there will be a lot of out of band signals mixed by multiples of the LO. This is especially true for strong local signals – I can see some strong FM station down-mixed from 100MHz to ~ 10Mhz

The entire board’s noise is very low – just 3-4dB over the sound card’s internal noise ( a Sound Blaster X-Fi )

Also, it’s input sensitivity is great – the antenna I’m using now it’s made of 3 loops of wire taped on the inside of a window ( 1.5m x 1m ).
The input balun is made with 3 windings of 10 turns of 0.25mm wire on a FT37-43 core. It works good enough for low frequencies.
It’s also good enough to receive aircraft-tower audio on 118.25Mhz with the indoor antenna.

For now I’m using a “dead bug style” filter for the LF bands, and no filter for frequencies over 25MHz – that area is usually silent enough not to need one.
I intend to make a set of helical filters for the amateur bands (21, 25, 28, 50 and 144Mhz) and another set for some weather satellite and air traffic.

The board is powered from 12V and with LT5517 enabled it uses ~ 120mA.

The entire project was make in Kicad.
The PCB’s where designed to fit a 100x50mm area – so i can get 2 boards in a single order. Also, they are single sided so I can solder them on a hot plate.
They where make by Pcb-Pool, on their prototype service and included a stainless steel solder mask for the top layer ( they provide them for top and bottom if there are components on both sides ). To reduce the cost I chose to skip the solder mask and silk screen. It was a good idea because they covered the entire board with a thin gold layer – Electroless Nickel Gold plating.

I’ve tried to keep the RF section as clean as possible and have an uninterrupted ground plane under the entire HF section. Also, I’ve included a lot of vias in the RF section.

Another part of the board is used as a heatsink for the 2 LDO’s that provide 10V to the op-amp and 5V to the LT5517. The one that generates the 5V rail has to dissipate (10v-5v)*100mA -> 0.5W. With the current board design both of them stay at ~ 40 deg. Celsius.

The next step will be to make a PLL LO generator for all band of interest, to free the RF generator. I’m thinking of a ADF4106 based one: microcontroller, ADF4106, 10MHz reference, loop filter and a switch matrix on the board – to switch power, control signals and RFin from a set of VCO modules ( VCO, attenuator to get to -10dBm and splitter ) – that’s another expensive project by itself.

I’ve also attached some audio samples, one of them has air traffic data, filtered and with the silence clipped out.
The other ones have amateur voice and packet ( PSK31/63, JT65 and RTTY )


Pdf schematic:

Kicad project:

Audio samples:
Air traffic, with a lot of silence:

Voice, 28MHz, a station from Spain

Voice on 28Mhz

Local JT65 – very strong and loud

Category: Electronica
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