Questions You Should Know about small signal schottky diode

I just got hold of a pack of capacitors for noise suppression on motors, and included is a 0.1uF capacitor for 'use with forward only speed controllers'. Can this replace a schottky diode? Or is it used in a different way?

Link to Hornby Electronic

Also - why are schottky diodes fitted to motors, and not the speed controller? Surely if a speed controller is forward only, it would have a built in diode?

Thanks in advance...

It could be a polar capacitor (like most electrolytic ones) which if inverse polarized is destroyed, hence only for forward ESC, but no real advantages in this case over the normal, non polar ones.

Caps and diodes do completely different things and are for different reasons:

A capacitor suppresses noise which could interfere on radio signal by smoothing out current peaks at commutator part switching, which are seen as sparks.

Shottky diode doesn't let a back-emf current generated at a motor which is braking or going reverse and acting as a generator destroy a fwd only ESC which cannot kope with inverse polarity/current.

Shottky diodes are probably fitted at the motors as they can be easily destroyed, so you can replace them easier.

Cheers

8-) I nearly put "Thanks (DJTheo) in advance"...

So there is little point in using it then?

How are the diodes easily destroyed? Do you mean physically? I understand if you connect it the wrong way round that they blow up, but surely they wouldn't be fitted the wrong way round if they were internal to the ESC. Is there another way of destroying them that I should look out for?

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Schottky diodes short out the back EMF like Theo says, that's why they are connected backwards across the usual voltage supply so they don't conduct normally (otherwise they'd pass about 10-20A easy).

Connect the Schottky wrongly (ie not backwards) and it'll conduct your power... and blow up. That's why you can't put them on reversing ESCs.

The back EMF is very high frequency (related to motor rpm) so the closer it is to the motor the better it is. Nothing to stop you putting it on the ESC or on the motor wires/plugs, the techies just say its "less efficient".

Then again Novak's "teabag" (a little PCB with schottkys) is made to be mounted remotely.

Some ESC's companies, to reduce the footprint, have no longer mounted the Schottky diode inside the ESC's case, many still do, hence no requirement to run a diode. The only reason that you would run a diode is that on some VERY low wind mod motors, the motor may cause a slight glitch on the steering servo, but only a MAY, not definate. Depends on the motor/esc/vehicle combo.

It is also intended to protect the fets in the esc and help to prolong the life of the esc. Supposedly there is an additional side effect in that it can help to prolong the life of the motor comm and the brushes.

You can run a diode if you want, but generally not needed on most of the better esc's. But I cant see any harm in adding one if it would help you to sleep at night. Personally I use schottky's on all my motors 11 turn and lower reguardless of the esc having one or not.

The lower the on-power resistance, the better the ESC copes with the instantaneous power requirements of a DC motor at stall (rest) when you decide you want to move. The older ESC's with todays motors will get at least a tad warm whereas the newer high end esc's wont even break a sweat. Due to this, and the FETS not getting warm, the motor/car/esc combo feels fresher, punchier and more responisive over the racer distance, instead of it feeling soft after a minute or two. Although this isn't the same for some of the other ESC's, hence the external Power Cap, which realy isn't a power cap, more a short burst of current to help the FET gates open and shut when they get warm due to cheap FETS. On a good esc the "NOS" cap does little or nothing.

http://www.teamassociated.com/misc/articles/article.745.htm

http://www.teamnovak.com/tech_info/more_info/schottky.htm

http://www.offshoreelectrics.com/why_to_use_diode.htm

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I am new to mixer circuit design, but I need to build a simple mixer that will downconvert a 10 GHz signal to a DC to 3 kHz signal. I know I can do this with a schottky diode and lowpass filter to isolate the lower harmonic (f-IF = f-RF - f-LO). The problem is that the RF signal will be in the uV range. Will mixing still work with a weak signal or do I need to amplify it? I read somewhere that the RF signal should be much lower than the LO signal such that the diode transconductance is affected by the LO signal only, but how low is too low?

Thanks for your help! Very difficult.

This would require a very elaborate receiver to convert this signal to a frequency where you could isolate one sideband.

You would have to buy most of it unless you have a lot of experience building equipment.

If you bought a receiver capable of receiving single sideband at 144 MHz, you could look for a converter that could mix your 10 GHz signal with a stable 10 GHz minus 144 MHz sine wave voltage.

You would be looking at at least $ to do that.

Just using a Schottky diode wouldn't work at all. You would need a very stable oscillator on 10 GHz and a filter at the same frequency that just doesn't exist.
And you would need a very high gain amplifier at 10 GHz. Also very difficult.

Then you need an antenna. The question of using an LO and schottky diodes to mix uV signals is something I have no experience with.

Mixing 10GHz to 0 and using a quadrature detector isn't that difficult, conceptually. But there are practical issues that complicate it.

You would need a 10GHz oscillator with very low phase noise in the +-3KHz band, which I think is a problem. Mixing to 144Mhz (as suggested, or some other frequency) means you need low phase noise out at 144Mhz, which is *maybe* more realistic. Then you can gain up and filter the 144Mhz before mixing to zero.

Remember that the mixer output will contain the +-3KHz components from both the LO and the signal.

Either you have to gain it up at 10GHz and then mix, or mix to an intermediate frequency, filter the image, gain it up, and mix to 0. I don't think I was specific enough. The entire setup would be composed of my ~10 GHz signal (which has already been received through a close range transmission from another circuit), the diode, and a 10GHz VCO. The intermediate frequency would be from DC to 3 kHz. The filter would then be applied at the output to get the lower sideband, which should be the easy part since the sidebands would be 20 GHz apart at the output. I'm not too concerned with output power as long as it's in the uV range at least. There are definitely diodes that move fast enough to achieve something like this. Heck, there are mixers that can do it that cost less than $40 or less. In fact, I have a microwave sensor that does the exact same thing I'm trying to do, and it costs $8. I just can't use it because I need to move my frequency wirelessly to another point in my design and DC-3 kHz is impossible to transmit on a small scale.

This is a low power project so I don't need tons of gain either. And the antenna is not the issue; I'm pretty sure I have that part down. Is my proposal still unfeasible? Technically the intermediate frequency is -3Khz to +3Khz.

The mixed down signal you see in DC to 3Hz would contain the information from the signal's 10Ghz +- 3KHz plus the LO's 10GHz +- 3KHz. You have -3KHz to +3Khz summed in the DC to 3KHz signal. Does that make sense?

If you want to receive only 1 sideband of the +-3KHz signal, You need a quadrature mixer and detector (I and Q). If you really don't want 3KHz single-sideband, then your only issue is LO phase noise in the +- 3KHz band and whether you can pull your signal out of that. I see, I wasn't accounting for negative harmonics. Well I'd rather not use a quadrature mixer if I don't have to, but it depends. See the actual frequency I am trying to oscillate will be fed into a frequency counter of some type and information will be extracted from it. Will the -3 kHz image hurt me too much when trying to do that? Sorry for all the questions. I'm actually still a student. Maybe you could clarify further.

What sort of modulation are you using?

How is your 10 GHz signal being generated? Is it a stable signal locked to a crystal oscillator?

Gain and selectivity are very difficult to obtain at microwave frequencies and not easy at 144 MHz.
So it is usual to use multiple conversion to get to a crystal filter at about 10 MHz which can deliver good selectivity.
In the conversion process, gain can be added to bring a 1 uV signal up to a volt or so where you can actually use it.

Most of this can be provided in the receiver (possibly 144 MHz because these are available) and a converter from 10 GHz to 144 MHz is very specialised but probably available.

A microwave detector for microwave ovens is a very different thing.
It operates on 2.4 GHz on millivolts or volts of signal.
It does not need to demodulate anything either. Whether the negative frequency aliasing helps or hurts depends on the nature of the signal you are trying to detect and the noisy environment it exists in.

What you are dealing with is not a "negative harmonic", but just an aliasing negative frequency. Think of a spectrum analyser view of the 10GHz signal. Replace the 10GHz with 0Hz. That is what you are creating with the mixer. Add LO feedthrough at 0 Hz, and LO phase noise. Then basically fold it over at zero. I expect LO noise and LO feedthrough will be the major issues with respect to the required signal level. You will have twice the noise bandwidth.

I have no idea how you will tune the LO to be right on to make this work. To the extent it drifts, the carrier drifts.

How fast are you trying to count? Is it just pulses (a signal level detector, basically)? I appreciate your replies.

I'm trying to build a speedometer for skis and longboards and the like that charts a user's speed by sending off off a signal and determining the velocity by reading the doppler-shifted return signal, like a radar gun only on a much smaller scale. I have a sensor that can do this; it transmits a ~10.525 GHz signal, receives the return signal, and mixes it with the same oscillator used for transmission (which is an integrated Dielectric resonator oscillator) to produce an IF signal that is at the doppler-shifted frequency, which should be DC to 6kHz.

The problem is, I need to output what the speed actually is on a wrist display, while the sensor will be mounted close to the ground, so I need to get the information wirelessly (hopefully) from the sensor to the display unit. To keep antennas small, I would like to transmit in the X-band, but the frequencies that contain the information will be DC-6kHz depending on how fast the user is going, hence, my downconversion predicament.

I'm also pretty inexperienced so forgive my naivety.

I think the sensor oscillators may be stable though and I may be able to access the onboard mixer in the device, which I could use to downconvert the signal for me. Otherwise, not sure how I would do it on my limited budget.