Why don't people use diff.pair for RF inputs ? - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: February 22, 2022, 02:58:48 am
content: I've seen numerous discrete and actual op-amp audio circuits, and they use a diff. pair or some diff. pair built into a current mirror type setup. But so far I've never seen an AM or FM receiver circuit, simple or brand name, that uses them. But they might use JFETs for high input impedance. But never a diff.pair for it's high input imp.Is there any reason why or have I just not seen them ?

author: MathWizard
author_rank: Super Contributor
timestamp: February 22, 2022, 11:07:43 am
content: Yeah I'm not up on transmission lines or the matrices for them yet. So I think I mean a singled ended signal, fed into 1 side of the diff. pair, with the other side tied to signal gnd or some Vref.I guess I've never never seen an all op-amp radio either.

author: jonpaul
author_rank: Super Contributor
timestamp: February 22, 2022, 11:21:41 am
content: Bonjour, a fully differential 88-108 MHz FM tuner was made in USA 1971..1976 Sequerra FM tuner was fully differential from antenna to output, using MC1496, Gilbert multipliers from Motorola.https://www.onsemi.com/pdf/datasheet/mc1496-d.pdfI worked with Dick Sequerra and Sid Smith (ex Marantz) to design this FM tuner.https://hometheaterreview.com/sequerra-model-1-fm-tuner-reviewed/The $2000...6000 machine became a classic and Collins FM broadcast monitor.The specs were unmatchable.Jon

author: CaptDon
author_rank: Super Contributor
timestamp: February 22, 2022, 02:12:28 pm
content: Differential inputs in the audio world are generally used with low source impedance microphones and at times 'line-in / line-out' feeds. They are referred to as 'balanced' above ground. This is a common way to reduce 'ground loop noise or interference' since the ground and any differences in ground potential from one end to the other are not part of the balanced signal composition. There is a term used called 'common mode rejection ratio' which can be understood as if you applied +1vdc to the high side of the balanced pair and also +1vdc to the low side a very small offset can occur at the output, usually reduced by about 100Db. In some cases as frequency goes up into the Mhz range it becomes difficult to maintain a perfect balance and that will create detectable differential mode artifacts and increase apparent noise. It is true there are 300 ohm and 600 ohm 'balanced feeders' or downleads and were VERY common with outdoor FM and television receiver antennas, HOWEVER, in about 99.9% of the cases that balanced line was immediately transformed to unbalanced at the input amplifier stage of the receiver by a 'balun' (balanced to unbalanced) transformer. There are also in broadcast something called UNUN's which are unbalanced to unbalanced and generally used to change impedance or for voltage isolation. Also note that a balun or unun adds loss to the signal chain. Remember that in the Mhz range even a 1 picofarad imbalance would defeat the benefits of a complex balanced circuit which also requires about twice the parts count and careful layout end to end.

author: MathWizard
author_rank: Super Contributor
timestamp: February 23, 2022, 01:06:35 am
content: Ok still don't think that's what I mean. In the most basic of receiver circuits, it's usually a LC tank, followed by a CE amp.So I'm just wondering, why I hardly ever, ever see this as an RF amp, whereas plenty of these old Tube and/or SS radio's would have this as the input to the audio stage.

author: Bud
author_rank: Super Contributor
timestamp: February 23, 2022, 01:11:17 am
content: Many RF ICs (e.g. mixers, DDS) have balanced input/output, typically fed via center tap RF transformers.

author: MathWizard
author_rank: Super Contributor
timestamp: February 23, 2022, 03:57:57 am
content: OK I have to learn some more lingo, I have a couple of radio fundamentals books I'm picking at. I have some laptop speakers, and I was going to make an amp and a single frequency TX/RX , so that I can listen to my computer from other rooms. So at 1st it might just be mono, AM, all single transistor stages, expect maybe the audio output.

author: radiolistener
author_rank: Super Contributor
timestamp: February 23, 2022, 06:04:08 am
content: A lot of RF IC has differential input/output. But usually coax cable is used as transmission line to connect radio receiver to antenna or one RF equipment to another. Coax cable is unbalanced transmission line, so it's not optimized for balanced differential input. Also there is needs to match coax line impedance with IC input impedance. For example coax line is 50 Ω but RF ADC input is usually 1-2 kΩ, so there is needs for impedance match between unbalanced coax cable and balanced IC input. This is why there is impedance match and balun circuits between device input and RF IC input.

author: Berni
author_rank: Super Contributor
timestamp: February 23, 2022, 06:27:24 am
content: There is differential signaling in RF but it is rare due to the issues versus benefits.It is more difficult to make a well performing diff pair cable for RF while a classical coax cable is simple, easy to make and performs well. Same applies to RF connectors. At the same time you don't really gain much noise immunity from doing differential in RF, the cables are many times longer than the wavelength they carry so signals travel along these cables as waves rather than conventional current flow as in audio cables. Ground loops are not a thing here.However there is a benefit to differential in RF signal processing as others have said. Inside chips and other small components the wavelengths are much longer than the distance between components so it behaves like conventional current just instead of KHz it is MHz or GHz. So going differential here can give you better noise immunity or cancellation of some effects. It is also easy to convert between differential and single ended at RF using a tiny transformer, so you will typically only convert the signal to differential right next to the chip that wants it to be differential. A lot of RF mixers actually differential even tho all the ports are single ended since they have transformers inside to do the differential conversion internally as part of there operating principle. SAW filters are also inherently differential but are often wire bonded out to the component leads as single ended.

author: CaptDon
author_rank: Super Contributor
timestamp: February 23, 2022, 02:36:14 pm
content: Well, maybe because the circuit you drew ISN'T a balanced circuit but merely a current mirror. Look at it...it is unbalanced in and unbalanced out. An RF amplifier designed for receiver use needs to have a low noise characteristic. All you did with your current mirror is add unneeded and unwanted parts. Do you see that it isn't a balanced circuit but only a current mirror circuit. You COULD take an A.C. coupled signal from both collectors and call it balanced but why add the extra complexity and also get a crappy noise figure as an extra unwanted bonus??

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2022, 10:58:38 pm
content: Quote from: MathWizard on February 22, 2022, 02:58:48 amDifferential transmission is more useful for a baseband signal where noise outside of the passband cannot be rejected by filtering. With a modulated signal, noise outside of the channel is largely irrelevant, so for instance common mode or ground noise gets rejected without differential signaling.On the RF input side, a true differential signal path has the disadvantage of doubling input noise. This also applies to oscilloscope probes; under conditions where there is no common mode noise to reject, single ended probes are inherently lower noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2022, 12:36:55 am
content: You do actually see that circuit sometimes, usually with the far input grounded (bypassed, as shown) and the output taken from the far side (noninverting) -- this avoids Miller effect, increasing gain-bandwidth while better isolating the input and output. Think of it as a partial inversion of the cascode, where instead of a common-emitter feeding a common-base, it's a common-collector (follower) feeding a common-base. (The other analogous inversion being the folded cascode, using complementary transistors; the sum of currents flows through a common bias resistor (rather than both transistors in series), much as the tail current is the sum of both transistor currents in the diff pair. Lastly there's also the complementary diff pair, which due to sharing the same current, behaves notably different, actually.)Not that you really see circuits much at all; everything's inside ICs nowadays. But if you want to be old school and build your own radios for example, yeah, it's good stuff.I think twin lead is unpopular in radio for, well nowadays its bulk, who wants to fiddle with fork terminals and ribbon-like cable probably? But also the two lines have more responsibility with respect to common mode, than with coax. Coax's CM can be hard shunted to ground via bulkhead connector; the best you can do with twin lead is a balanced transformer (balbal, or balun if your internal circuitry is unbalanced after all), so if you need DC coupling, you're probably in for a hard time.Which is the case for one digital comm standard: Ethernet. Now-(mostly?)-deprecated ISDN too, though the current (A)DSL also follows a (roughly) similar interface, I think. In all cases, the transformer isolation permits use of cheap unshielded twisted pair, while also isolating hazardous surge voltages.Going back to the circuit --Electrically speaking, there isn't really anything fundamental between a common-emitter and diff pair, for example. Diff pair is just two B-E's in series, so it has twice the input impedance and input-referred noise* of C-E. But if you can match that impedance with a transformer anyway, what's a 1.4:1 turns ratio matter between friends?*Twice under RMS, which is to say, sqrt(2) more voltage.Or similarly for JFETs or what have you. Which are most popular, I think, because they have reasonable specs of course (your average junk JFET has a higher operating frequency than your average junk BJT, compare 2N3819 to 2N3904 for example), and as a grounded-gate amplifier, the source input impedance (~ 1/gm) is pretty handy to match to the transmission line (1/(20mS) = 50Ω).(Not that 2N3819 has that much gm, but 2N4043, BF862, etc. do.)(Note that BJTs are limited by fT, i.e., the gain drops off pretty sharply around that frequency, as an inherent device characteristic. You simply need better -- faster -- types to go to higher frequencies. JFETs may be limited on bandwidth, but not center frequency: a 200MHz BJT probably won't do anything in a 600MHz tuned amplifier, but a JFET will do just fine there.)Note that, although the diff pair circuit has reasonable CMRR (common mode rejection ratio), this is only true to the extent that:- The tail current remains constant (over all important CM frequencies),- The output / load (collector circuit) is matched and also differential, and- The input voltage remains within limits.The last one is the real killer. Unshielded cable can pick up thousands of volts quite easily (say from ESD transients), literally killing semiconductor circuits if directly connected. Even just a few volts is enough to exceed the supply rails in most circuits, corrupting the signal. And if you need to prevent that, then you have to put in a shield anyway. And if you need a shield (example: USB, which uses shielded twisted pair), it's kind of like... well now why did you go to all that trouble of making a whole-ass shielded twin-lead cable, when you could just use one, and make it a hell of a lot cheaper and off-the-shelf -- i.e. standard coax?Well, for USB specifically, they do need enough bandwidth and SNR (for High Speed mode) that differential really is worthwhile, plus they need a custom cable anyway to carry power and ground; on the upside, the shield doesn't need to be terrific quality, usually a modest braid around a foil screen and that does it.Also compare to 10BASE-2 ("thinnet"), the coax version of Ethernet (and other contemporary standards), whereas 10BASE-T (and so on, as we use today) uses twisted pair. These are rather slower than modern Ethernet of course, but the low bandwidth was, well, certainly suitable at the time of course, and made up for that with reasonable range, while still offering good EMI immunity. (Note that thinnet was still transformer isolated -- a necessity to deal with ground loop as well as transients -- but it was done internally at the digital interface! Probably another good reason UTP took over, if you need the transformers anyway, why not put them on the media instead?)There's also RS-485, a differential line standard, typically used for serial data on industrial networks; it doesn't have ~kV of CM range, so some error detection is needed to ride through large upsets like ESD. For more modest levels of noise (up to about 10V peak, typical of nearby portable radiotransmitters), it offers excellent CMRR -- the voltage range extends beyond the supply rails (0 to 3.3/5V) by use of resistor dividers at the input. So with modest filtering, some protection for robustness, and some error correction/detection in the protocol, it can be quite reliable.Tim

author: cdev
author_rank: Super Contributor
timestamp: April 02, 2022, 02:59:20 am
content: Look at this about Canada. Canada is nice. Its a great place. You should check them out.https://www.canada.ca/en/immigration-refugees-citizenship/news/2022/03/canada-to-welcome-those-fleeing-the-war-in-ukraine.html