Re: [Discuss-gnuradio] comparing SDR upconverters, thermal stability
Hi Daniel,
shortly answering your questions:
On 30.03.2016 17:50, Daniel Pocock wrote:
> OK, so keeping the mixer cooled will help reduce loss but has nothing
> to do with frequency stability?
yes; well, the point is that the mixer is really just a multiplier,
built from a nonlinear device, and we use the fact that this
nonlinearity introduces a term that contains the product of the two
input signals, and usually filter out the unwanted original signals, and
wrong intermodulations afterward.
Temperature might change the coefficients of whatever model you use to
describe the nonlinear element;
for example, the current I(U) through a Diode is well-modeled with an
exponential function
$I(U) = I_0 e^{\alpha U}$,
and you can represent $e^x$ as power series
$e^x=\sum_{n = 0}^{\infty} \frac{x^n}{n!} = 1 + x + \frac{x^2}{2!} +
\dots$ ,
and abort that series after the second part. You'll notice the quadratic
component in there; and if $U = \frac x\alpha$ happens to be the sum of
two cosines, so
$x=\frac{\cos(f_1 t)+ \cos(f_2 t)}{\alpha}$ (imagine $f_1$ is the
frequency of your soon to be mixed up signal, and $f_2$ your
oscillator's frequency)
by the binomials, you'd have $x^2=\frac{\cos(f_1 t)^2 + 2\cos(f_1
t)\cos(f_2 t)+\cos(f_2 t)^2}{\alpha^2}$ with the cosine product in the
middle; now $\frac{2\cos(f_1 t)\cos(f_2
t)}{\alpha^2}=\frac{2}{\alpha^2}\cos(f_1 t)\cos(f_2 t)$ is only
sensitive to temperature in $\alpha$, not in either $f_i$, so
temperature will change the amplitude of your modulated output, but not
the frequency (lest temperature changes $\alpha$ so fast that you'd
notice the amplitude change as some frequency component; but we can
safely assume it won't).
For completeness: If you use Euler's formula to break down cosine,
you'll find that
$\cos(f_1t)\cdot\cos(f_2t)=\frac14\left[\cos\left((f_1t)-(f_2t)\right)+\cos\left((f_1t)+(f_2t)\right)\right]=
\frac14\left[\cos\left((f_1-f_2)t\right)+\cos\left((f_1+f_2)t\right)\right]$
which contains the sum of the two frequencies, $f_1+f_2$, which is what
we wanted for upconversion!
Now, the oscillator, on the other hand, does change its frequency
> That is definitely quite a relevant point, where I live at present the
> indoor temperature can often be a lot lower than indoor
Hm, the point of cozyness was more to avoid temperature changes;
basically, that's why OCXOs are used: you can relatively well control
the temperature of an oscillator inside a small oven; first of all,
building a control loop to keep that temperature constant is relatively
easy, because a big-enough oven won't cool down or heat up too quick,
and secondly, physics is cooperating here by the laws of thermodynamics.
>
>> That brings one down to the question whether you have the chance to use
>> the same oscillator for both your SDR device and the upconversion; ...
> For people using low-cost RTL-SDR dongles that might be more than they
> can expect
Well, there are people who use one RTL-SDR's clock for multiple
RTL-SDRs, to keep them frequency aligned, so that's probably possible if
you can solder some kind of amplifier (a FET will possibly do, but watch
out for intermodulation products ;) ) to original oscillator.
>
> For the proper SDR boards (e.g. USRP, BladeRF, HackRF) is this feasible?
USRPs: well, given you have either a USRP that integrates its RF
frontend (B2xx, E3xx), or a daughterboard that can transmit: you could
directly generate a tone for mixing with at arbitrary frequencies within
the frontend's frequency range, and that tone would be derived from the
same frequency reference as the oscillator frequency to convert down in
the receiver.
Some USRPs (mainly, the X3x0) also export their 10MHz reference (but on
those, you could also just use a BasicRX daughterboard that doesn't have
any mixer and just receive from e.g. 50MHz in baseband).
BladeRF: same thing; if you can transmit, you can probably also generate
a tone. I haven't worked with that; if it's not full-duplex capable, you
might have to make some firmware/software modifications to be able to
continously have such a TX tone.
HackRF: definitely not Full-Duplex.
Cheers,
Marcus
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Re: [Discuss-gnuradio] comparing SDR upconverters, thermal stability
If you can synthesize the LO for the upconverter using the RTL XO as a reference, the drift may tend to cancel. I say "may" as this is a trick used in dual or triple down conversion chains. If you can alternate between low and high sided IFs, then the drifts will move in the opposite directions (but not the same amount). Though with the complexity of doing that, might as well just lock everything it to a 10 MHz reference. Maybe just periodically calibrate to WWV. Lou Daniel Pocock wrote > Has anybody been using any of the upconverters for SDR and has anybody > made any comparison of them? -- View this message in context: http://gnuradio.4.n7.nabble.com/comparing-SDR-upconverters-thermal-stability-tp59194p59204.html Sent from the GnuRadio mailing list archive at Nabble.com. ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] comparing SDR upconverters, thermal stability
Tsys is essentially irrelevant for HF receivers, since Tambient is much, much higher (thousands of K) than even some really-poor RF engineering scenarios. At HF, galactic background can be very high--1e4K or more. On 2016-03-30 10:30, Marcus Müller wrote: > Hi Daniel, > > haven't made experience with any of these upconverters; but: > > The really temperature-sensitive aspect of an upconverter is probably > the oscillator, not the mixer. So the trick might really be keeping > your upconverter in the same environment as your SDR receiver (assuming > both don't have overly well temperature-compensated or oven-controlled > oscillators), as that will just as much limit your frequency accuracy. > > Mixer circuits do exhibit conversion loss that tends to get worse with > rising temperature, but that'll not distort your signal much. > > The core question here is whether you'll deterioriate your system > performance if you keep your upverter far from your antenna; my guess is > that you'd have an LNA close to the antenna, anyway, so keeping the > upverter's oscillator warm and cozy near your SDR device won't be that > complicated, probably. > > That brings one down to the question whether you have the chance to use > the same oscillator for both your SDR device and the upconversion; that > way, you'd only have to worry about one device drifting under any > circumstance. Does your device give you the chance to couple out a clock > or maybe transmit a sine? > > Best regards, > Marcus > > On 30.03.2016 09:18, Daniel Pocock wrote: > >> Hi all, Has anybody been using any of the upconverters for SDR and has >> anybody made any comparison of them? I've seen some comments suggesting that >> many of the low cost models have poor thermal stability[1], has anybody seen >> problems with this in practice for receiving modes like SSB on amateur HF >> bands? If this is an issue, is anybody aware of alternative models that are >> more robust? Regards, Daniel 1. >> http://www.rtl-sdr.com/review-of-the-spyverter-upconverter/#comment-79953 >> [1] ___ Discuss-gnuradio mailing >> list Discuss-gnuradio@gnu.org >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [2] > > ___ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [2] Links: -- [1] http://www.rtl-sdr.com/review-of-the-spyverter-upconverter/#comment-79953 [2] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] comparing SDR upconverters, thermal stability
On 30/03/16 16:49, mle...@ripnet.com wrote: > All of them use reasonably good crystals or crystal XOs. Certainly a > lot better than the RTLSDRs most of these were intended for. > > I assume that you're talking about frequency stability, rather than gain > stability? > Yes, frequency stability. SSB voice signals in the HF spectrum may be around 3000 Hz wide ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] comparing SDR upconverters, thermal stability
On 30/03/16 16:30, Marcus Müller wrote: > Hi Daniel, > > haven't made experience with any of these upconverters; but: > > The really temperature-sensitive aspect of an upconverter is probably > the oscillator, not the mixer. So the trick might really be keeping > your upconverter in the same environment as your SDR receiver (assuming > both don't have overly well temperature-compensated or oven-controlled > oscillators), as that will just as much limit your frequency accuracy. > > Mixer circuits do exhibit conversion loss that tends to get worse with > rising temperature, but that'll not distort your signal much. > OK, so keeping the mixer cooled will help reduce loss but has nothing to do with frequency stability? > The core question here is whether you'll deterioriate your system > performance if you keep your upverter far from your antenna; my guess is > that you'd have an LNA close to the antenna, anyway, so keeping the > upverter's oscillator warm and cozy near your SDR device won't be that > complicated, probably. > That is definitely quite a relevant point, where I live at present the indoor temperature can often be a lot lower than indoor > That brings one down to the question whether you have the chance to use > the same oscillator for both your SDR device and the upconversion; that > way, you'd only have to worry about one device drifting under any > circumstance. Does your device give you the chance to couple out a clock > or maybe transmit a sine? > For people using low-cost RTL-SDR dongles that might be more than they can expect For the proper SDR boards (e.g. USRP, BladeRF, HackRF) is this feasible? ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] comparing SDR upconverters, thermal stability
I know that some of them, like the HAMITUP, use a socket for the XO, which one could easily replace with a TCXO. Check with the manufacturers about the XO spec they use. On 2016-03-30 11:43, Daniel Pocock wrote: > On 30/03/16 16:49, mle...@ripnet.com wrote: > >> All of them use reasonably good crystals or crystal XOs. Certainly a lot >> better than the RTLSDRs most of these were intended for. I assume that >> you're talking about frequency stability, rather than gain stability? > > Yes, frequency stability. SSB voice signals in the HF spectrum may be > around 3000 Hz wide > > ___ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [1] Links: -- [1] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] comparing SDR upconverters, thermal stability
All of them use reasonably good crystals or crystal XOs. Certainly a lot better than the RTLSDRs most of these were intended for. I assume that you're talking about frequency stability, rather than gain stability? On 2016-03-30 03:18, Daniel Pocock wrote: > Hi all, > > Has anybody been using any of the upconverters for SDR and has anybody > made any comparison of them? > > I've seen some comments suggesting that many of the low cost models have > poor thermal stability[1], has anybody seen problems with this in > practice for receiving modes like SSB on amateur HF bands? > > If this is an issue, is anybody aware of alternative models that are > more robust? > > Regards, > > Daniel > > 1. http://www.rtl-sdr.com/review-of-the-spyverter-upconverter/#comment-79953 > [1] > > ___ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [2] Links: -- [1] http://www.rtl-sdr.com/review-of-the-spyverter-upconverter/#comment-79953 [2] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] comparing SDR upconverters, thermal stability
Hi Daniel, haven't made experience with any of these upconverters; but: The really temperature-sensitive aspect of an upconverter is probably the oscillator, not the mixer. So the trick might really be keeping your upconverter in the same environment as your SDR receiver (assuming both don't have overly well temperature-compensated or oven-controlled oscillators), as that will just as much limit your frequency accuracy. Mixer circuits do exhibit conversion loss that tends to get worse with rising temperature, but that'll not distort your signal much. The core question here is whether you'll deterioriate your system performance if you keep your upverter far from your antenna; my guess is that you'd have an LNA close to the antenna, anyway, so keeping the upverter's oscillator warm and cozy near your SDR device won't be that complicated, probably. That brings one down to the question whether you have the chance to use the same oscillator for both your SDR device and the upconversion; that way, you'd only have to worry about one device drifting under any circumstance. Does your device give you the chance to couple out a clock or maybe transmit a sine? Best regards, Marcus On 30.03.2016 09:18, Daniel Pocock wrote: > > Hi all, > > Has anybody been using any of the upconverters for SDR and has anybody > made any comparison of them? > > I've seen some comments suggesting that many of the low cost models have > poor thermal stability[1], has anybody seen problems with this in > practice for receiving modes like SSB on amateur HF bands? > > If this is an issue, is anybody aware of alternative models that are > more robust? > > Regards, > > Daniel > > > 1. http://www.rtl-sdr.com/review-of-the-spyverter-upconverter/#comment-79953 > > > ___ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Discuss-gnuradio] comparing SDR upconverters, thermal stability
Hi all, Has anybody been using any of the upconverters for SDR and has anybody made any comparison of them? I've seen some comments suggesting that many of the low cost models have poor thermal stability[1], has anybody seen problems with this in practice for receiving modes like SSB on amateur HF bands? If this is an issue, is anybody aware of alternative models that are more robust? Regards, Daniel 1. http://www.rtl-sdr.com/review-of-the-spyverter-upconverter/#comment-79953 ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
