How does the image rejection filter in the R820T(2) work?

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sivan_toledo
Posts: 1
Joined: Sun Oct 15, 2017 3:57 pm

How does the image rejection filter in the R820T(2) work?

Post by sivan_toledo » Sun Oct 15, 2017 4:10 pm

Reading about the R820T and R820T2, there is something that I find difficult to understand: how do they achieve a variable-frequency image-rejection filter with a -65dB attenuation and very sharp transition?

Suppose, for example, that I tune an RTL-SDR to 903.57MHz. The IF is 3.57MHz so the LO is tuned to 900MHz (assuming a low-side injection) and the bandpass is about 902.5 to 904.5. At those frequencies, the image reject filter needs to have almost no attenuation. From about 900-4.57 to about 900-2.57 (up to 897.43MHz), the image reject filter should have a rejection of 65dB; this is how I understand the data sheet. The filter should be this deep for about 2 or 2.5 MHz (more for 6-8MHz DVB-T transmissions) and should transition to no attenuation a couple of MHz higher.

Is this how things work? Do they really achieve this kind of image rejection and steep transitions?

I ask because I always thought that it is hard to achieve high image rejection when the image and target frequencies are close, and that this is why many receivers have a high first IF, at 45 or 70 MHz, to move the image far out so that it can be filtered easily.

Any insights will be appreciated.

Thanks, Sivan

on1bes
Posts: 21
Joined: Sat Dec 24, 2016 11:03 pm

Re: How does the image rejection filter in the R820T(2) work

Post by on1bes » Thu Oct 19, 2017 9:45 pm

There is NONE . It's a simple DVB-T TV stick you known.

hotpaw2
Posts: 28
Joined: Sat Jan 14, 2017 11:07 pm

Re: How does the image rejection filter in the R820T(2) work

Post by hotpaw2 » Sat Oct 21, 2017 1:27 am

So why is 65dB image rejection mentioned in the data sheet specification?

snn47
Posts: 62
Joined: Tue Dec 27, 2016 11:00 pm

Re: How does the image rejection filter in the R820T(2) work

Post by snn47 » Sat Oct 21, 2017 2:58 pm

Suppose, for example, that I tune an RTL-SDR to 903.57MHz. The IF is 3.57MHz so the LO is tuned to 900MHz (assuming a low-side injection)
If you assume a assuming a high-side injection (1.8 GHz to 3 GHz) the BP would not be required.

https://www.rtl-sdr.com/using-the-rtl-s ... ansmitter/

Here are the meassured values from an SA (VCO range 1.770 - 3.54 GHz).
http://www.steila.com/blog/index.php?co ... &id_post=9

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