a 70db amp for a noise source - Page 1
Forum section: electronics / projects

author: zibadun
author_rank: Regular Contributor
timestamp: December 26, 2012, 06:56:31 pm
content: I've got a white noise source based on a zenner diode (N-Gen made by Elecraft) which gives -110dbm or so output. I need to boost the output another 65 to 70 db over a range of 1-60 mhz. Would an amp based on LMH6503http://www.ti.com/lit/ds/snosa78d/snosa78d.pdfset for +70db gain work for this purpose?

author: McMonster
author_rank: Frequent Contributor
timestamp: December 26, 2012, 07:10:03 pm
content: The values given on the first page of the datasheet are for optimistic conditions. Take a look at "Gain Adjustment Range" in electrical characteristics.

author: zibadun
author_rank: Regular Contributor
timestamp: December 26, 2012, 08:12:16 pm
content: Quote from: McMonster on December 26, 2012, 07:10:03 pmit says 68 db over 0-30 mhz which is OK with me. I've ordered some samples.

author: JackOfVA
author_rank: Unknown
timestamp: December 27, 2012, 01:37:56 am
content: 70 dB gain in a single stage of LMH6503 at 60 MHz?That's not how I read the data sheet. For a gain of 100V/v (40 dB) the bandwidth is 50 MHz (-3dB) and for 70 dB gain the bandwidth is going to be much, much less.It will also be interesting to build a single stage amp with 70 dB gain at RF frequencies. It will likely turn into an oscillator instead of an amplifier.I would be inclined to use a cascade of 3 Mini-Circuits GaLi-74 MMIC amplifiers. 50 ohms in/out, quite stable with even a halfway decent layout on the PCB and not all that expensive. Gain will be around 70 dB with 3 in series and the frequency range will be determined by your choice of components and PCB layout, as each amp is good to 1 GHz. 1-60 MHz is trivial with these devices.Jack

author: zibadun
author_rank: Regular Contributor
timestamp: December 27, 2012, 04:11:07 am
content: That is what happens when a newb tries to design an amp.I suspected 70db would be too much gain for one stage. The microcircuits monolitic amp looks a lot easier with fewer components and only a single power supply. Thanks for the advice Jack!

author: zibadun
author_rank: Regular Contributor
timestamp: January 01, 2013, 03:04:22 am
content: I picked up a few MAR-1 MMIC amps on ebay and connected three of them in series. I thought they are very similar to Gali 74. I'm getting overall gain of 22db on the white noise. The strange thing is as soon as I add a new stage the previous stage gain drops dramatically. Any idea what I can be doing wrong?Happy New Year all!

author: Shuggsy
author_rank: Regular Contributor
timestamp: January 01, 2013, 07:47:09 am
content: What DC bias are you using for the amps? If I read that correctly, you have 220 ohm resistors for the bias resistor, so is it around 9VDC per the datasheet (I'm assuming it'sthese amps from Mini-Circuits)? To be optimally biased, you'll need to dial in the voltage to match your 220 ohm bias resistor, otherwise your performance may not meet the specs on the datasheet. However, I don't think that's your main issue.I don't see any RF chokes on the DC feed lines, so your RF may be leaking on to your DC feed path. Put some inductors in as RF chokes as shown in the example circuit in the datasheet. You want an inductor value that gives you a high impedance at your max frequency of interest... something above 10uH should give nice rejection. Try adding something and see what the results for a single stage look like. If the single-stage performance is about the same as it was (or better!) prior to the RF choke, cascade it to the next stage with an RF choke on its bias as well. If all is working correctly, you should see ~17dB gain per stage based on the datasheet.

author: zibadun
author_rank: Regular Contributor
timestamp: January 01, 2013, 01:29:47 pm
content: hi shuggsy.yes they are 220s, powered from a 9v battery. I'll try to add chokes and see what happens .. May be they are not so "optional". regards