On 07/03/2017 09:15 PM, Clifford Heath wrote:> On 02/07/17 16:46, upsidedown@downunder.com wrote: >> On Sun, 2 Jul 2017 09:08:05 +1000, Clifford Heath <no.spam@please.net> >> wrote: >>> On 02/07/17 07:14, upsidedown@downunder.com wrote: >>>> On Fri, 30 Jun 2017 17:09:50 +1000, Clifford Heath >>>> <no.spam@please.net> wrote: >>>>> The hobbyist community seems blissfully unaware of many of >>>>> these problems, >>>> The problem is that in past decades quite complex structures were >>>> needed for spectrum analyzers and general coverage receivers (such as >>>> multiple PLLs). Today, when good components are available, such as >>>> NCOs, DACs and ADCs the same performance can be achieved with less >>>> complexity and hence less cost. >>> Not the same performance. Adequate for many tasks, but no way the same. >> The weakest point in both SA as well as general coverage receivers and >> scanners is the mixer. > > Mixers are non-linear devices.^^^^^^^^^^ A small quibble, because this bit of Bad Info keeps popping up. A perfect mixer is a bilinear device: the IF output is linear in both the LO and RF signals. If you put some DC in the LO port, the RF->IF path satisfies all the requirements for a linear, time invariant network. > Even a perfect mixer will still have the > same image problems that plague real receivers. As you say, the image exists even with a perfectly linear multiplier, because of the identity 2 sin A sin B = cos(a-b)-cos(a+b). Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
LF Spectrum Analyser
Started by ●June 27, 2017
Reply by ●July 4, 20172017-07-04
Reply by ●July 5, 20172017-07-05
On 05/07/17 03:00, Phil Hobbs wrote:> On 07/03/2017 09:15 PM, Clifford Heath wrote: >> On 02/07/17 16:46, upsidedown@downunder.com wrote: >>> On Sun, 2 Jul 2017 09:08:05 +1000, Clifford Heath <no.spam@please.net> >>> wrote: >>>> On 02/07/17 07:14, upsidedown@downunder.com wrote: >>>>> On Fri, 30 Jun 2017 17:09:50 +1000, Clifford Heath >>>>> <no.spam@please.net> wrote: >>>>>> The hobbyist community seems blissfully unaware of many of >>>>>> these problems, >>>>> The problem is that in past decades quite complex structures were >>>>> needed for spectrum analyzers and general coverage receivers (such as >>>>> multiple PLLs). Today, when good components are available, such as >>>>> NCOs, DACs and ADCs the same performance can be achieved with less >>>>> complexity and hence less cost. >>>> Not the same performance. Adequate for many tasks, but no way the same. >>> The weakest point in both SA as well as general coverage receivers and >>> scanners is the mixer. >> >> Mixers are non-linear devices. > ^^^^^^^^^^ > > A small quibble, because this bit of Bad Info keeps popping up. A > perfect mixer is a bilinear device: the IF output is linear in both the > LO and RF signals.I don't think a physical realisation exists at VHF or higher? Maybe for *very* small signals. It's not really relevant what's possible under 1MHz, LM13700 etc. Clifford Heath.
Reply by ●July 5, 20172017-07-05
On Tuesday, July 4, 2017 at 8:55:38 PM UTC-7, Clifford Heath wrote:> On 05/07/17 03:00, Phil Hobbs wrote:> > A small quibble, because this bit of Bad Info keeps popping up. A > > perfect mixer is a bilinear device: the IF output is linear in both the > > LO and RF signals.> I don't think a physical realisation exists at VHF or higher? > Maybe for *very* small signals.> It's not really relevant what's possible under 1MHz, LM13700 etc.It's not challenging, but for those of us who chose our modulations, it's interesting. It's as relevant as we choose to make it.
Reply by ●July 5, 20172017-07-05
On 05/07/17 14:55, whit3rd wrote:> On Tuesday, July 4, 2017 at 8:55:38 PM UTC-7, Clifford Heath wrote: >> On 05/07/17 03:00, Phil Hobbs wrote: > >>> A small quibble, because this bit of Bad Info keeps popping up. A >>> perfect mixer is a bilinear device: the IF output is linear in both the >>> LO and RF signals. > >> I don't think a physical realisation exists at VHF or higher? >> Maybe for *very* small signals. > >> It's not really relevant what's possible under 1MHz, LM13700 etc. > > It's not challenging, but for those of us who chose our modulations, > it's interesting. It's as relevant as we choose to make it.Sorry. My comment on relevance was in the context of upsidedown's "SA as well as general coverage receivers and scanners". Not too many of those care about <1MHz.
Reply by ●July 5, 20172017-07-05
On Tue, 4 Jul 2017 11:15:07 +1000, Clifford Heath <no.spam@please.net> wrote:> >There's quite a good review of available devices in the slides here >(though these still pass over some of the problems): ><http://microhams.blob.core.windows.net/content/2017/03/RTL-SDR-dongle.pdf>Now I understand your negative attitude towards SDR, if you use that article as a reference :-) Those dongle DVB-T chips are mainly intended for upper-VHF/lower-UHF applications in which noise figures are critical, but the signal levels are usually not too bad. Using it down to HF and lower (directly or via an upconverter) will cause signal handling problems. The only 8 bit ADC forces the use of some preamplifier with AGC in front of the ADC. The small dongle size doesn't allow much front end selectivity. I am thinking more about SDRs with Tayloe mixer/detector followed by 16-24 bit "audio" ADCs. The Tayloe mixer/detector consists of four S&H units each fed with 90 degree phase shift from each other, two differential amplifier generating I and Q signals. These are typically implemented with a 1 of 4 bus switch and a dual op-amp. The I and Q signals can then be processed in analog or digital domain. If processing is done in digital domain, the I and Q channel phase shift and amplitude variations can be quite well be compensated to give goo opposite sideband suppression.
Reply by ●July 5, 20172017-07-05
On 05/07/17 17:34, upsidedown@downunder.com wrote:> On Tue, 4 Jul 2017 11:15:07 +1000, Clifford Heath <no.spam@please.net> > wrote: > >> >> There's quite a good review of available devices in the slides here >> (though these still pass over some of the problems): >> <http://microhams.blob.core.windows.net/content/2017/03/RTL-SDR-dongle.pdf> > > Now I understand your negative attitude towards SDR, if you use that > article as a reference :-) > > Those dongle DVB-T chips are mainly intended for upper-VHF/lower-UHF > applications in which noise figures are critical, but the signal > levels are usually not too bad. Using it down to HF and lower > (directly or via an upconverter) will cause signal handling problems. > > The only 8 bit ADC forces the use of some preamplifier with AGC in > front of the ADC. The small dongle size doesn't allow much front end > selectivity. > > I am thinking more about SDRs with Tayloe mixer/detector followed by > 16-24 bit "audio" ADCs. The Tayloe mixer/detector consists of four S&H > units each fed with 90 degree phase shift from each other, two > differential amplifier generating I and Q signals. These are typically > implemented with a 1 of 4 bus switch and a dual op-amp. > > The I and Q signals can then be processed in analog or digital domain. > If processing is done in digital domain, the I and Q channel phase > shift and amplitude variations can be quite well be compensated to > give goo opposite sideband suppression.You still haven't told me anything I didn't already know.
Reply by ●July 5, 20172017-07-05
On 07/04/2017 11:55 PM, Clifford Heath wrote:> On 05/07/17 03:00, Phil Hobbs wrote: >> On 07/03/2017 09:15 PM, Clifford Heath wrote: >>> On 02/07/17 16:46, upsidedown@downunder.com wrote: >>>> On Sun, 2 Jul 2017 09:08:05 +1000, Clifford Heath <no.spam@please.net> >>>> wrote: >>>>> On 02/07/17 07:14, upsidedown@downunder.com wrote: >>>>>> On Fri, 30 Jun 2017 17:09:50 +1000, Clifford Heath >>>>>> <no.spam@please.net> wrote: >>>>>>> The hobbyist community seems blissfully unaware of many of >>>>>>> these problems, >>>>>> The problem is that in past decades quite complex structures were >>>>>> needed for spectrum analyzers and general coverage receivers (such as >>>>>> multiple PLLs). Today, when good components are available, such as >>>>>> NCOs, DACs and ADCs the same performance can be achieved with less >>>>>> complexity and hence less cost. >>>>> Not the same performance. Adequate for many tasks, but no way the >>>>> same. >>>> The weakest point in both SA as well as general coverage receivers and >>>> scanners is the mixer. >>> >>> Mixers are non-linear devices. >> ^^^^^^^^^^ >> >> A small quibble, because this bit of Bad Info keeps popping up. A >> perfect mixer is a bilinear device: the IF output is linear in both the >> LO and RF signals. > > I don't think a physical realisation exists at VHF or higher? > Maybe for *very* small signals. > > It's not really relevant what's possible under 1MHz, LM13700 etc. >A Gilbert cell made from 80 GHz SiGe:C bipolars should be able to do linear multiplication up to ~10 GHz. With unmatched transistors it would take a bit of tweaking, but they cost 25 cents each in small quantity. Normally Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 5, 20172017-07-05
On 05/07/17 22:42, Phil Hobbs wrote:> On 07/04/2017 11:55 PM, Clifford Heath wrote: >> On 05/07/17 03:00, Phil Hobbs wrote: >>> On 07/03/2017 09:15 PM, Clifford Heath wrote: >>>> On 02/07/17 16:46, upsidedown@downunder.com wrote: >>>>> On Sun, 2 Jul 2017 09:08:05 +1000, Clifford Heath <no.spam@please.net> >>>>> wrote: >>>>>> On 02/07/17 07:14, upsidedown@downunder.com wrote: >>>>>>> On Fri, 30 Jun 2017 17:09:50 +1000, Clifford Heath >>>>>>> <no.spam@please.net> wrote: >>>>>>>> The hobbyist community seems blissfully unaware of many of >>>>>>>> these problems, >>>>>>> The problem is that in past decades quite complex structures were >>>>>>> needed for spectrum analyzers and general coverage receivers >>>>>>> (such as >>>>>>> multiple PLLs). Today, when good components are available, such as >>>>>>> NCOs, DACs and ADCs the same performance can be achieved with less >>>>>>> complexity and hence less cost. >>>>>> Not the same performance. Adequate for many tasks, but no way the >>>>>> same. >>>>> The weakest point in both SA as well as general coverage receivers and >>>>> scanners is the mixer. >>>> >>>> Mixers are non-linear devices. >>> ^^^^^^^^^^ >>> >>> A small quibble, because this bit of Bad Info keeps popping up. A >>> perfect mixer is a bilinear device: the IF output is linear in both the >>> LO and RF signals. >> >> I don't think a physical realisation exists at VHF or higher? >> Maybe for *very* small signals. >> >> It's not really relevant what's possible under 1MHz, LM13700 etc. > > A Gilbert cell made from 80 GHz SiGe:C bipolars should be able to do > linear multiplication up to ~10 GHz.Linear in what signal levels? Don't you have to stay under a couple of mV? It's not normal to drive a Gilbert cell that way. I have some HFA3101's coming, which are only 10GHz Ft, but then I only really care about mixing <1GHz. I don't have gear to test the result, but a friend does. Will see how I go. Clifford Heath.
Reply by ●July 5, 20172017-07-05
>Linear in what signal levels? Don't you have to stay under a couple >of mV?A real Gilbert cell is a current-mode device--the inputs drive current mirrors (what the LM13700 data sheet calls "input diodes"). That ideally gets rid of the tanh nonlinearity.>It's not normal to drive a Gilbert cell that way.A lot of the time hard-switching is better because diode-bridge and poorly-designed Gilbert-style mixers generate most of their IMD during the transitions and because hard switching gets rid of the AM noise of the LO. It would be fun to build a BFP840 Gilbert cell and see how it worked. Based on their Spice models they should have excellent log conformity. Cheers Phil Hobbs
Reply by ●July 5, 20172017-07-05
