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

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sivan_toledo
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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
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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
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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
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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

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

Post by Brightnoise » Sun Jan 21, 2018 9:36 pm

There is a valid specification for image rejection. The LO generates two signals with about 90 degrees phase difference. This is called a complex oscillator signal. The antenna input signal is converted in TWO parallel mixers and generates TWO IF-signals with also 90 degrees phase shift. (The image frequency of this conversion process is on the same IF-frequency but the 90 degrees offset is just the other direction, or minus 90 degrees shift. In popular words: positive and negative frequencies). These two signals are fed to the AD-converters. The combination in the digital baseband chip takes care for the specified image rejection. This is again a complex frequency shift, decimation and finally IF low-pass filters for the selectivity. The analog phase errors (not exact 90 degrees) are the IQ-errors (and the correction system in the baseband chip can, for a large part, even correct phase and amplitude errors made in the input chip). Thats how it works.

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

Post by Brightnoise » Mon Jan 22, 2018 10:21 am

Correction: the above explanation is valid for the E4000 tuner. The used tuner R820T has an analog combination filter in the output of the two parallel analog mixers. After this "all-pass 90-degrees combination filter" the SINGLE ended IF signal is fed to ONE of the two AD-converters. As designer I should go for TWO IF-branches and correction in the digital domain. It is very difficult to guarantee better image rejection than 40 or 50 dB in such an analog system. Apparently they succeeded.

W1ABA
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Re: How does the image rejection filter in the R820T(2) work?

Post by W1ABA » Fri Jul 06, 2018 7:38 pm

WOW, the contents of this thread are right on!!

In broader terms, the chips used in the RTL-SDR were never intended to be used at VLF, LF, HF and VHF. And they weren't made for reception of weak signals.

To the best of my knowledge, the newer units have 2 small smt inductors, which is the full extent of any filtering. SMT inductors aren't exactly good performers::>

Enjoy, I'm new to the forum and I'm loving it!

1ABA

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