Hi all

Can someone please provide me with some guidance? We are working on a

passive radar project and the aim is to implement a pcl radar receiver

on the USRP1 using TVRx daughterboards. I have the Microtune 7901.

I did some tests to see what the effects of the RF and IF amplifiers

in the tuner are (linear region,etc.). I noticed that the maximum

output of the TVRx is -15dBm regardless of the input signal and the

gains. For this measurement, I’ve got the test circuit that they have

in the Microtune datasheet with the Minicircuits RF transformer to

match the 200 ohm output of the TV tuner to 50 ohms. Also, I captured

a signal from the TVRx and looked at it in Matlab. The maximum I

value is 17 and the maximum Q value is 16, which means that we are not

close to using the whole dynamic range.

I also did a test with the BasicRx db. This time I injected a 13dBm

signal into RX-A. The maximum value in Matlab is 7919 which looks

much better.

Now I have 2 questions:

-Does anyone know anything about the TVRx and why I only get an output

of -15dBm? The Microtune datasheet is a bit vague. It only says that

the signal level for 1dB RF gain compression is 72dBuV (-113dBm) with

AGC deactivated. Maybe the TVRx wasn’t designed to utilise the entire

dynamic range of the ADC??

-What is the maximum signal level after the DDC+filtering,etc? My

guess is around 2047*8 because of the bit-shifting at the ADC

interface.

Sebastiaan

–

Sebastiaan H.

Radar and Remote Sensing Group, University of Cape Town

Tel: +27 83 305 5667

Sebastiaan H. wrote:

in the Microtune datasheet with the Minicircuits RF transformer to

match the 200 ohm output of the TV tuner to 50 ohms. Also, I captured

a signal from the TVRx and looked at it in Matlab. The maximum I

value is 17 and the maximum Q value is 16, which means that we are not

close to using the whole dynamic range.

There are a couple of things to note here. First, the 200 ohm output of

the microtune is plenty to drive the ADCs which have an input impedance

close to 1K. So using a transformer to step the impedance down to 50

ohms gives you a loss of 6dB (2x voltage).

Second, At 0dB of PGA gain, the maximum input to the ADCs is 2Vpp

differential, which is 10dBm in a 50 ohm system. The programmable gain

amplifier gives you 20dB of gain, so with the gain at max, the input

needed to clip would be -10dBm, which is about right for the max output

from the TVRX.

I also did a test with the BasicRx db. This time I injected a 13dBm

signal into RX-A. The maximum value in Matlab is 7919 which looks

much better.

13dBm will clip the ADC. 10dBm is the real max full scale value.

Now I have 2 questions:

-Does anyone know anything about the TVRx and why I only get an output

of -15dBm? The Microtune datasheet is a bit vague. It only says that

the signal level for 1dB RF gain compression is 72dBuV (-113dBm) with

AGC deactivated. Maybe the TVRx wasn’t designed to utilise the entire

dynamic range of the ADC??

With the PGA, it uses most of the dynamic range.

Matt

Matt

Sorry to bother. Please tell me why I lose 6dB when measuring the

signal at pins 17 and 18? I have the RF transformer from Minicircuits

that is supposed to match the 200ohm output of the TVRX to a 50ohm so

that I can measure it on a spectrum analyser. Because it is matched,

I should not have a 6dB loss?

I believe the input impedance of the ADC is around 220ohms? How does

the BasicRx work? The input is 50ohms, so is there an RF transformer

on the BasicRx that matches this to the ADC?

Thanks in advance.

Sebastiaan

Which can easily be seen if you realize that in ideal

Passive transformer no power is added or lost.

Remember : P = (U*U)/R

No let’s use it for the transformer:

Pin = (Uin*Uin)/200*

Pout = (UoutUout)/50

Passive no loss transformer:

Pin = Pout

So:

(Uin*Uin)/200 = (Uout*Uout)/50

<=>

(Uout*Uout) = (Uin*Uin) * (50/200) = (Uin*Uin)/4

So:

Uout = Uin/2

There you have the factor 0.5!

Kees (W1/PE1LDW)

Sebastiaan H. wrote:

Matt

Sorry to bother. Please tell me why I lose 6dB when measuring the

signal at pins 17 and 18? I have the RF transformer from Minicircuits

that is supposed to match the 200ohm output of the TVRX to a 50ohm so

that I can measure it on a spectrum analyser. Because it is matched,

I should not have a 6dB loss?

While the transformer does not lose power, you have to understand that

the ADC is a voltage input device. A transformer that converts 200

ohms to 50 ohms divides the voltage in half. When we say that the full

scale level is 10 dBm, it actually means 2V peak to peak. Once you put

it into the transformer, you’ll only have 1V peak to peak.

I believe the input impedance of the ADC is around 220ohms? How

does

the BasicRx work? The input is 50ohms, so is there an RF transformer

on the BasicRx that matches this to the ADC?

It is a voltage input. We match the input with a 50 ohm terminating

resistor.

Matt