[USRP-users] Frequency translation works differently on N210 and B210.

Ian Buckley ianb at ionconcepts.com
Thu Oct 2 01:32:58 EDT 2014


What you are seeing is *exactly* as sampling theory would predict.
Your fc+f signal shows up on your spectrum analyzer at 800.625Mhz (Remember your 750kHz became 750*1.6666/2.000)
Your "fc-f" image is not at fc-f (which would be 799.375MHz) but at 1.66666Mhz - 0.625Mhz = 798.95Mhz

You see the alias so prominently because the pure CIC filter has terrible stop band rejection which is why it's largely unsuitable as a generic interpolation filter on it's own.
Sweep you CW frequency slowly and watch the direction the image appears from…this is your clue where it's coming from, and a good practical technique when looking at real world signals and there unexpected by-products.
If you select peak hold on your spectrum analyzer and sweep the frequency you will plot the frequency response of the CIC filter. Try it with sample rate 2.5MHz and 1.25MHz and see the response of the small half band filter and then the cascaded half band filters

Now if you turn up the gain some more you will see a true "fc-f" signal, probably some 40-50db below your main signal and this will be due to the residual IQ imbalance present in real world direct conversion receivers.

-Ian


On Oct 1, 2014, at 12:45 PM, Urban Hakansson <uhakansson at tecore.com> wrote:

> Yes, I deliberately requested a sample rate that would force the use of the FPGA CIC filters. Fs = 1.666667 MHz is fine. I expected roll-off but I did not expect the replica at fc-f. The CIC filters should not introduce that kind of error, at least that is my understanding.
> 
> ----- Original Message -----
> From: "Ian Buckley" <ianb at ionconcepts.com>
> To: "Urban Hakansson" <uhakansson at tecore.com>
> Cc: "Marcus D. Leech" <mleech at ripnet.com>, usrp-users at lists.ettus.com
> Sent: Wednesday, October 1, 2014 2:53:04 PM
> Subject: Re: [USRP-users] Frequency translation works differently on N210 and B210.
> 
> Did you note the warnings that this generates? The USRP's can not perform fractional rate conversions. Your sample rate was coerced to be compatible with a 5MHz master clock rate and up sampling was pure CIC filter based which has less than ideal spectra:
> 
> 
> UHD Warning:
>    The requested interpolation is odd; the user should expect CIC rolloff.
>    Select an even interpolation to ensure that a halfband filter is enabled.
>    interpolation = dsp_rate/samp_rate -> 3 = (5.000000 MHz)/(2.000000 MHz)
> 
> UHD Warning:
>    The hardware does not support the requested TX sample rate:
>    Target sample rate: 2.000000 MSps
>    Actual sample rate: 1.666667 MSps
> 
> UHD Warning:
>    The requested interpolation is odd; the user should expect CIC rolloff.
>    Select an even interpolation to ensure that a halfband filter is enabled.
>    interpolation = dsp_rate/samp_rate -> 3 = (5.000000 MHz)/(2.000000 MHz)
> 
> UHD Warning:
>    The hardware does not support the requested TX sample rate:
>    Target sample rate: 2.000000 MSps
>    Actual sample rate: 1.666667 MSps
> 
> 
> On Oct 1, 2014, at 11:41 AM, Urban Hakansson <uhakansson at tecore.com> wrote:
> 
>> Ian, 
>> 
>> Thanks for taking a look at this problem. There is no sample rate conversion 2MHz->5MHz taking place. I executed the same flow graph for two different sample rates. The master clock rate was set to 5 MHz in both cases.
>> 
>> Case 1) The sample rate was set to 5 MHz (equal to the master clock rate). 
>> 
>> Case 2) The sample rate was set to 2 MHz (not equal to the master clock rate).
>> 
>> Urban
>> 
>> ----- Original Message -----
>> From: "Ian Buckley" <ianb at ionconcepts.com>
>> To: "Urban Hakansson" <uhakansson at tecore.com>
>> Cc: "Marcus D. Leech" <mleech at ripnet.com>, usrp-users at lists.ettus.com
>> Sent: Wednesday, October 1, 2014 2:20:34 PM
>> Subject: Re: [USRP-users] Frequency translation works differently on N210 and B210.
>> 
>> Urban, a quick question…I just glanced at your flow graph, it's very simple…I'm wondering where did the sample rate conversion occur (2MHz->5MHz) in your example 2) quoted below?  There is no re-sampling block instantiated in your flow graph.
>> 
>> On Oct 1, 2014, at 10:33 AM, Urban Hakansson via USRP-users <usrp-users at lists.ettus.com> wrote:
>> 
>>> First, thank you for responding so promptly. I attach the Gnu Radio flow-graph and the generated python script. It contains two USRP sink objects, one configured for the B210 and the other for the N210. FYI, I use GnuRadio companinion 3.7.1.1 and 3.7.2.2.
>>> 
>>> I also attach two example images from the spectrum analyzer looking at the spectrum from the B210.
>>> 
>>> Image 1) B210 master clock rate = 5MHz, Sampling rate = 5 MHz, frequency of complex exponential = 1.2 MHz, center frequency = 800MHz.
>>> No replica at fc-f. As expected.
>>> 
>>> Image 2) B210 master clock rate = 5MHz, Sampling rate = 2 MHz, frequency of complex exponential = 750 kHz, center frequency = 800MHz.
>>> Replica at fc-f.
>>> 
>>> The result from the N210 is a replica at fc-f as well so I don't include a image.
>>> 
>>> Regards,
>>> 
>>> Urban Hakansson
>>> 
>>> ----- Original Message -----
>>> From: "Marcus D. Leech via USRP-users" <usrp-users at lists.ettus.com>
>>> To: usrp-users at lists.ettus.com
>>> Sent: Tuesday, September 30, 2014 8:11:34 PM
>>> Subject: Re: [USRP-users] Frequency translation works differently on N210 and B210.
>>> 
>>> On 09/30/2014 06:20 PM, Urban Hakansson via USRP-users wrote:
>>>> Hi everybody,
>>>> 
>>>> I have a problem. I include below 1) General information, 2) Introduction to my problem 3) Detailed description of my problem.
>>>> 
>>>> General background information about my environment:
>>>> Fedora 17 Linux; GNU C++ version 4.7.0 20120507 (Red Hat 4.7.0-5); Boost_104800; UHD_003.007.001-0-unknown
>>>> 
>>>> N210 informationfrom uhd_usrp_probe:
>>>> | Device: USRP2 / N-Series Device
>>>> | _____________________________________________________
>>>> | /
>>>> | | Mboard: N210r4
>>>> | | hardware: 2577
>>>> | | mac-addr: 00:80:2f:0a:e6:15
>>>> | | ip-addr: 192.168.2.199
>>>> | | subnet: 255.255.255.255
>>>> | | gateway: 255.255.255.255
>>>> | | gpsdo: none
>>>> | | serial: F4A09C
>>>> | | FW Version: 12.4
>>>> | | FPGA Version: 10.1
>>>> 
>>>> B210 information from uhd_usrp_probe:
>>>> | Device: B-Series Device
>>>> | _____________________________________________________
>>>> | /
>>>> | | Mboard: B210
>>>> | | revision: 4
>>>> | | product: 2
>>>> | | serial: F571B5
>>>> | | FW Version: 4.0
>>>> | | FPGA Version: 3.0
>>>> 
>>>> 
>>>> Introduction/background: It is my understanding that outputting a real-valued baseband signal x(t) = a*sin(2*pi*f*t) on an RF carrier fc should result in two components at fc+f and fc-f, but outputting a complex-valued baseband signal x(t) = exp(2*pi*f*t) should only result in an fc+f component. The complex exponential can be used for frequency translation but is causing me serious problems on the N210 + SBX daughterboard.
>>>> 
>>>> Detailed Problem Description: Using GnuRadio I output a simple baseband complex exponential at frequency +f centered at the RF center frequency fc. Now on the N210 in addition to the sinusoid at fc+f there is a unexpected replica at fc-f about 13-14 dB below fc+f. When I run the same script on the B210 and set master clock rate equal to the sample clock rate, I only see the desired frequency component at fc+f. There is no replica at fc-f as in the case of the N210. This is the correct behaviour as I understand it. However, if I don't set the master clock rate equal to the sample rate on the B210 I do get the undesired replica at fc-f 13-14 dB below the signal at fc+f just as I did on the N210.
>>>> 
>>>> Why does it only work if the master clock rate is set equal to the sample clock rate on the B210? I have read somewhere on the mailing list that the FPGA including CIC and HB filters are bypassed in this case.
>>>> 
>>>> Question: How can I output a simple complex-valued baseband signal x(t) = exp(2*pi*f*t) on an RF carrier fc on the N210 and only get an fc+f component so I can use this mechanism to perform frequency translation?
>>>> 
>>>> Thanks for you consideration.
>>>> 
>>>> Regards,
>>>> 
>>>> Urban Hakansson
>>>>                                                                                                                                                                                                                                      This e-mail may contain privileged, confidential, copyrighted or other legally protected information, and is intended exclusively for the intended recipient.  If you are not the intended recipient (even if the e-mail address above is yours), you may not review, store, use, copy, disclose or retransmit it in any form.  If you are not the intended recipient or otherwise have received this by mistake, please immediately notify the sender by return e-mail (or sysadmin at tecore.com), then delete the message in its entirety. Thank you.
>>>> 
>>>> _______________________________________________
>>>> USRP-users mailing list
>>>> USRP-users at lists.ettus.com
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>>> Could you perhaps share your Gnu Radio flow-graph with us?  It would 
>>> help in seeing where your problems might originate.
>>> 
>>> 
>>> 
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>> 
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> 
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