# USRP Power Supply Noise **URL:** \ **Category:** GNU Radio\ **Created:** [August 24, 2010, 4:35pm UTC](https://www.ruby-forum.com/t/usrp-power-supply-noise/193800 "2010-08-24T16:35:45Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![jfl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ee59a6/32.png) [@jfl](https://www.ruby-forum.com/u/jfl)\ **Post date:** [August 24, 2010, 4:35pm UTC](https://www.ruby-forum.com/t/usrp-power-supply-noise/193800/1 "2010-08-24T16:35:45Z")
Hello, I am attempting to build an analog front-end to a USRP 1 device. I have selected a VCO that will suit the frequency range we are looking at: It is an RFVC1800 and has a tuning range of approximately 7.4 - 12.4 GHz over a 13 volt tuning voltage. As this VCO is very sensitive to external noise, some power filtering will be necessary to minimize jitter and phase noise. What I would like to know is whether or not there are any other adverse effects from using the simple switch mode supply that is included with the USRP? ## Thanks, ~Jeffrey L. ~Jeffrey L., K1VZX ---
**Author:** ![jfl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ee59a6/32.png) [@jfl](https://www.ruby-forum.com/u/jfl)\ **Post date:** [August 24, 2010, 4:57pm UTC](https://www.ruby-forum.com/t/usrp-power-supply-noise/193800/2 "2010-08-24T16:57:25Z")
On Tue, Aug 24, 2010 at 9:32 AM, Jeffrey L. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hello, > I am attempting to build an analog front-end to a USRP 1 device. I have > selected a VCO that will suit the frequency range we are looking at: It is > an RFVC1800 and has a tuning range of approximately 7.4 - 12.4 GHz over a 13 > volt tuning voltage. As this VCO is very sensitive to external noise, some > power filtering will be necessary to minimize jitter and phase noise. What > I would like to know is whether or not there are any other adverse effects > from using the simple switch mode supply that is included with the USRP? Are you referencing the power brick that comes with the USRP (and provides the raw +6V DC input)? If so, you’ll definitely want to do some significant filtering/cleaning up of this supply, and want to think carefully about the gnd paths to your VCO as well. IIRC, we saw non-trivial ringing on the gnd that had a period of ~5 uS and occurred every 20 mS (don’t quote me on the exacts here, as my memory is a bit fuzzy, though this is what I sketched into my engineering notebook at the time). We ultimately determined we were simply seeing the residual effects of the switching power supply in the power brick and had to live with it or use a better external power suply. Most daughterboards take the raw +6V passed to it right from the power brick, and the use LDOs on the daughterboard to generate the voltages necessary for the sensitive RF components. But this is typically stepping down to 5V/3.3V, which, with a 6V input, provides enough margin for an LDO to work and still provide excellent power supply ripple rejection (PSRR). This is key when generating a stable supply for feeding a VCO, as any variation (i.e. ripple) will show up as variation in the output of your VCO. If I were you, I’d start with a good oscilloscope and begin probing around the power supply lines and the gnd lines of the USRP to see what the supplies look like, and from there you should be able to assess whether or not you’ll be able to use this voltage as a starting point for your design. You’ll also want to make sure the gnd reference point for the scope is as close to the measurement point as possible to ensure that you aren’t picking up stray signals/interference. Good luck, and keep the list posted of progress. Sounds like quite a challenging project 🙂 – John O. CEO/System Architect Epiq Solutions [www.epiq-solutions.com](http://www.epiq-solutions.com) ---
**Author:** ![jfl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/ee59a6/32.png) [@jfl](https://www.ruby-forum.com/u/jfl)\ **Post date:** [August 26, 2010, 11:05pm UTC](https://www.ruby-forum.com/t/usrp-power-supply-noise/193800/3 "2010-08-26T23:05:29Z")
On 08/24/2010 07:32 AM, Jeffrey L. wrote: > Thanks, > ~Jeffrey L. The 6V supply on the daughterboards is filtered, but it is still unregulated, so it will rise and fall between 5.8 and 6.4V with load changes. You’ll want to pass it through an LDO to use it. You mention 13V, which will take a switcher to boost the 6V. In that case, you can use the 6V directly into your switcher, which should then be followed by an LDO to your final voltage. Matt