Wideband amp wanted for a noise  generator to feed my SA - Page 1
Forum section: electronics / rf-microwave

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 22, 2018, 02:31:21 pm
content: I want to use the noise generator I have to tune a band pass filter on my SA. What will it need amplifying to be usable? Has anyone a link to a simple to make amp to get the signal from around 100kHz to about 1Mhz up to a suitable level please? Also what does ENR mean in the spec sheet attached please, and what output is this thing supposed to have on its own? SA is HP8568B Thanks

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 22, 2018, 05:53:58 pm
content: I would think you could get flat gain to 1MHz with a decent op amp. For higher frequencies there are MMIC gain blocks, but they might not have very flat gain.ENR is Excess Noise Ratio. It is approximately the difference in noise power in dB between the noise source powered on and off. This gives the exact definition in section 3:http://literature.cdn.keysight.com/litweb/pdf/5952-8255E.pdfTurned off, it would have an output noise power at room temperature of about -174dBm/Hz. So if ENR is 25dB, then turned on the output noise power would be approximately -174dBm/Hz + 25dB = -149dBm/Hz.

author: jpb
author_rank: Super Contributor
timestamp: February 22, 2018, 08:19:57 pm
content: It is a long time since I worked with MMICs but if you're looking for wide-band gain then Distributed or Travelling Wave amplifiers will give this - back in the 80s we had chips that went from DC to 18GHz.I don't know if they are commercial products though.A random search came up with this:https://datasheets.globalspec.com/ds/2844/AvagoTechnologies/15282232-A3AA-486E-827E-78BE3BC49BA2

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 22, 2018, 11:23:05 pm
content: I'm curious - why would a regular old LNA not work for this application? What goes wrong when you try to amplify a noise source with a LNA? There are tons of LNAs with gains of up to 40dB out there for not very much money.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 23, 2018, 03:43:18 pm
content: Thanks for the replies, let's get specific though, would this do the job please?https://www.ebay.co.uk/itm/122562329344

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 23, 2018, 07:25:18 pm
content: Quote from: Chris Wilson on February 23, 2018, 03:43:18 pmAccording to the datasheet, your HP8568B has a noise floor of about -130dBm/Hz at 100 kHz and -150dBm/Hz above 1MHz.This amplifier will add about 35dB of noise, so that brings the noise source + amplifier up to about -149dBm/Hz + 35dB = -114dBm/Hz.So you will have about 16dB of dynamic range at 100kHz to 36dB of dynamic range at 1MHz and above.So you should be able to see something with this amplifier, but you may want more gain if you need more dynamic range.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 23, 2018, 11:28:05 pm
content: Thanks again. As a lowly intermediate licence holder I now have to ask, will mi likely need more dynamic range to tune a filter to block mainly uk high power broadcast band interference from the LF ham band which is 136 to 138khz? The main big power UK station is BBC Radio 4 on 198khz. Thanks

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 24, 2018, 12:11:37 am
content: I'm not a ham, but I guess if you can see your wanted signal and the unwanted signal on the spectrum analyzer, the difference in level between the two should give you an idea of how much suppression you need.

author: chris_leyson
author_rank: Super Contributor
timestamp: February 24, 2018, 01:01:16 am
content: A simple PRBS generator , pseudo random binary sequence generator, would do the job, just clock some simple shift register logic at 50MHz or so and low pass filter the output, job done.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 12, 2018, 08:38:04 pm
content: Hi Chris,I think the amp used here is an INA02186.I have built a lot of preamps with this one and they work perfectly. And 32dB gain at the low end is quite good.https://electronicprojectsforfun.wordpress.com/rf-module-gallery/the-amplifier-module-gallery/mmic-amplifiers/an-ina02186-mmic-amplifier/

author: Wolfgang
author_rank: Super Contributor
timestamp: August 14, 2018, 09:20:24 am
content: I agree, but its not so easy to achieve a wide flat frequency range.I built some, if interested look here:https://electronicprojectsforfun.wordpress.com/making-noise/digital-noise-sources-i-have-built/

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 14, 2018, 10:27:02 am
content: Thanks for the further replies, I am using 2 of these in series to up the signal from the commercial noise source and my SA (HP8568B) is now working well to see my band pass filter efforts. I am also (possibly naively) using one as a pre amp for my Red Pitaya LF 137kHz low frequency ham band receiver. It seems to work well and I use a low noise adjustable voltage source to set the gain as needed. Some good info thank you everyone for the repliesEDIT: Two of what, put the damned link in Chris, you dozy pratthttps://www.ebay.co.uk/itm/0-1-2000MHz-RF-wideband-amplifier-gain-30dB-low-noise-amplifier-LNA-TK/263840684787?hash=item3d6e215af3:g:l98AAOSwZ4dZNQOFApologies, getting old....