SDRx: Which LNA chip?


#1

Hello,

I have some old SDRx PCBs. They perform very well on other frequencies with another SAW filter. However, I have only done that for L-band (GPS L1).

I am thinking of doing the same for a UHF frequency. Could you chould share the part number of the LNA chip so I can see if that is possible and maybe some passives also need to be changed?

It is Rev 1v06.1

Thank you!


#2

I was thinking to simply reflow the old SAW filter off and bypassing it. Of course that provides 10 dB or so more gain and lots of unwanted signal.

Now, if you can find replacement saw filters… There’s plenty of room in the can to glue it down upside down and solder leads to it. But if we can find SAW filters that for example span the VHF or UHF ham band … that would be great.

–Konrad


#3

Thanks.

I have already replaced the saw filter in two of the SDRx boards to operate on the GPS L1 frequency. This works very well. I used a hot air gun to reflow the SAW in place of the original one (the footprint is quite standard).

Bypassing the filter does not work, the R820 tuner is saturated with unwanted out of band signals (GSM1800 mostly). It is the same as when you connect an LNA to a normal rtl-sdr stick without any filter in between.

I was just wondering if the LNA (without the filter) is broadband, or if the chip is already intended as an L-band LNA (eg, a GNSS frontend). With a UHF SAW filter, would the receiver work well on a UHF frequency in its passband without changing any other components?

SAW filters for the 70cm ham band should be available. At least for the ISM portion of it I have held them in my hand. I did a quick search at Tai-Saw and found this one: TA0693A. It covers the 430-440MHz 70cm band :slight_smile: You can find some stores on the internet selling it (4-6$), but getting a sample should also be possible. The footprint is not the same as the SAW currently installed in the SDRx (frequency is lower so device is bigger), but it will fit on the footprint to the left of it:

VHF saw filters for the HAM band I have never seen. They will be quite large also. This is a primary exclusive allocation for HAM so there is likely little commercial interest. You can have a custom SAW made for a competitive price if you buy a large enough amount (eg, group order, …)


#4

Ahhh this is a V1.04 board. I have later revisions V1.06 and V1.07 (not here with me right now) What’s in the can looks different.

How do they select the filter?

Yes, without a filter, the SDR front-end will get overloaded.

The chip is very broadband ~60MHz to 1.6 GHz if my memory serves me right.

Nice. There are loads of them for different bands on E-Bay.

The TA0693 covers a good part of the 75cm band.

I would probably prototype an external shielded >–[LNA]—[VHF SAW]—[LNA]–> and feed the output into the LNA_BYPASS input

–Konrad


#5

the LNA chip is narrow band, peaking around the GPS application bands


#6

@Zoltan,

Thanks, this is what I feared. I will use a normal rtl-sdr then.

@Konrad_Roeder

The SAW is selected by mounting 0ohm resistors or, even better, capacitors with SRF close to the operating frequency over the unsused pads.

Indeed, the later SDRx versions don’t have the large SAW footprint, only the one where the original SAW is mounted on, so this limits you to SAWs for >800MHz or so, but since we learned the LNA is narrowband this is enough.