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Question about Variable Gain Amplifier

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Member level 1
Nov 29, 2002
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ad8346 differential input p-p

Does anyone know a low noise Variable Gain Amplifier which is able to amplify the 900 - 1500 MHz band?

Thank you in advance!


a better way

I do not know of one. You can use a variable attenuator before or after a fixed gain amplifier. Putting it before degrades the noise but allows high signal levels. Putting it after has minimum noise but the amplifier may saturate on high signal levels.


Could you please advise me the manufacturer of such attenuators?

Or may be you can give me an advise on the overall problem. I am designing direct convesion modulator. I have I/Q baseband, Local Oscilator (900-1500MHz), Quadrature Modulator.
I need to be able to control the modulator output gain. Which solution would be better:
1. Put two Variable Gain Amplifiers in the baseband for each of the I/Q component before the Quadrature Modulator?

2. Or put one variable attenuator after the modulator?

Thank you!


put them before

Put the attenuators before the modulator. I need more information to advise you. If the gain is to be set once your best bet is step atenuators. What is the source, amp0litude, and frequency range of your modulating signals?

Yes you are right. The gain should be set once and should change rarely.

The I and Q will pass the AD9763 10bit-DACs, then Low Pass Filters (pass band 0-10MHz), then (possible) step attenuators and then AD8346 Quadrature Modulator (output 900-1500 MHz).

According to AD8346 modulator overview for differential I/Q inputs: the drive signal should be a 2 V p-p (differential) signal with a bias level of 1.2 V, that is, each input swings from 0.7 V to 1.7 V.

The maximum modulator output power is 0dBm. I need to control this in the range between -20dBm to 0 dBm.
As I know the AD9763 DAC has "GAINCTRL MODE" and "REFERENCE CONTROL AMPLIFIER" but I am not sure that I can use it to control the gain. May be I am wrong and the problem is much simpler then I think.

Please let me know what do you think.


it looks like

It looks like you can control the current out of the DAC and then you can control the conversion of current to voltage with an op amp circuit. Therefore, you certainly can set the output level. You can also add the DC offset you need at the op amp stage. It may be easier to convert to a voltage before filtering since the DAC output is differential. Although, you can easily use just one side as an output and ignore the voltage across the other side's resistor.

If this is a small project, it would probably be easier to just calculate the resistor values you need and solder them in instead of using some sort of adjustable resistor which may not work well at your frequeny (bit rate) range.

Hi ,

I think, it will be better to use a single RF attenuator at the output.
It can be made easy with p-i-n diodes. Adding atenuators in I & Q channels will affect your output SNR.



optimum point

There is an optimum level to baseband drive the I/Q modulators. Too high will distort the signal and make it wider in spectrum. Too low will make the carrier suppression less compared to the modulation sidebands. Therefore, set the baeband drive level as a trade off between your two needs and then adjust the final output to the level you want with either a fixed attenuator or an amplifier.

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