SDR dongle - how "hot" can the input be?


#1

When using a low noise amplifier, how hot can the input to the RTL-SDR (R820T2) be (in dBm?) I can’t find any specs on this chip or the device.


#2

There’s a spec sheet for the R820T floating around on the internets – marked preliminary 2011 – Google is your friend. It says the max input power is +10 dBM.

Executive summary: Just because the max input power is +10 dBm doesn’t mean it will work with high level signals present; it just means it won’t be damaged.

Discussion:

The big print giveth and the fine print taketh away… When we look at IIP3 numbers:

Max Input Power +10 dBm
IIP3 at LNA Max Gain -7.5 dBm
IIP3 at LNA Min Gain +35 dBm

This tells me the little beastie can take +10 dBm without damaging the input stages. But depending on the input (LNA) gain, the nonlinear nature of those input stages will produce all sorts of mixing products (with significant energy in the baseband) from the signals coming in.

(here comes the rant)

This is why the forums are filled with reports of: “I went to a better antenna and my numbers went down,” “I added an amplifier and my numbers went down,” “I changed my high-gain whiz-bang semi-sweet CoCo with hazelnuts antenna for a ground plane and my numbers went up,” and the like. Above a (gain dependent) threshold level, higher level input signals (away from the frequency of interest) mean more spurious mixing products, and decreased performance. Lower signal levels, particularly away from the frequency of interest, keep those unwanted mixing products low.

And that’s why some of us get all freaky about filters… The LNA I use (Mini Circuits ZRL-2300; I have a few of them) has an IP3 of +42 dBm and a noise figure at 1 GHz of around 2 dB with a measured gain of around 24.8 dB. The input is filtered and the output runs through a 1090 MHz EPCOS SAW filter so the only thing the SDR gets is signals around 1090 MHz.

(end of rant)

bob k6rtm


#3

Thanks Bob


#4

Do you have a picture of your EPCOS saw filter setup? When I look up this part, it shows a small surface mount device. It’s hard to imagine how you integrated it into your setup.


#5

You’re correct, it’s a surface mount device. It’s mounted dead bug style on a small piece of double-sided FR4 with a female SMA connector at each end. The connections are made using 30 gauge wire and are very short. After testing in this state, I put a piece of Kapton tape over the connections, and then a piece of copper foil tape soldered to the back side of the pcb to effectively enclose (and shield) the whole thing. From there, it’s just another thing with SMAs on it.

bob k6rtm


#6

Wow. I’m tempted to make a custom PCB at OshPark.


#7

Bob
When I looked at the spec sheet for the filter it seems you have both grounds set to the antenna center. Is that ok?

Scotty W7PSK


#8

Bob,
Where did you order your SAW?

Cheers!
LitterBug


#9

eBay –

http://www.ebay.com/itm/B1602-SAW-filters-Epcos-1090MHz-ADS-B-AirNav-SBS-/110619017914

price has gone up a bit since my last order!

bob k6rtm


#10

While the SAW is designed for balanced operation, running it single-ended isn’t a problem.

Pins 1 and 2 are the input, 5 and 6 the output. I’m feeding pin 1 and taking the output from pin 5. Pins 2, 3, 4, 6, 7, 8 are grounded. One jumper goes from the pcb across 8 and 4 to the pcb on the other side of the chip. Shorter jumpers do 2, 3, to the pcb, and 6, 7 to the pcb. The Network Analyzer says it works, and gives me pictures that look a lot like the spec sheet…

Yes, a pcb would be nice, but if I’m going to do that, I’ll include the LNA, and everything has to be controlled impedance. I’m also still scrounging Mini Circuits surface-mount filters for a front end; so far the “big” can SHP-1000 seems to be the best performer.

bob k6rtm