# Ultra high frequency oscillators

Started by December 27, 2016
```Could some electronics guru please shed some light on this ?

How is the carrier wave for high frequency
wireless communication transmitters generated ?
Specifically, any oscillator would need
negative impedance device, e.g., a bipolar
transistor to sustain the oscillations.
So, what transistors would be used in e.g.,
a GSM transmitter with a carrier frequency of 1.8
GHz

```
```On Tuesday, December 27, 2016 at 7:21:24 AM UTC+1, daku...@gmail.com wrote:
> Could some electronics guru please shed some light on this ?
>
> How is the carrier wave for high frequency
> wireless communication transmitters generated ?
> Specifically, any oscillator would need
> negative impedance device, e.g., a bipolar
> transistor to sustain the oscillations.
> So, what transistors would be used in e.g.,
> a GSM transmitter with a carrier frequency of 1.8
> GHz
>

1.8GHz is not ultra high frequency

Cheers

Klaus
```
```On Mon, 26 Dec 2016 22:21:15 -0800 (PST), dakupoto@gmail.com wrote:

>Could some electronics guru please shed some light on this ?
>
>How is the carrier wave for high frequency
>wireless communication transmitters generated ?
>Specifically, any oscillator would need
>negative impedance device, e.g., a bipolar
>transistor to sustain the oscillations.
>So, what transistors would be used in e.g.,
>a GSM transmitter with a carrier frequency of 1.8
>GHz
>
>

An oscillator is simply a noise amplifier with frequency selective
positive feedback. The amplifier gain needs to be higher than the
losses in the feedback network.

```
```On Tue, 27 Dec 2016 02:50:13 -0800 (PST), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>1.8GHz is not ultra high frequency

According to ITU

UHF = Ultra High Frequency = 300 .. 3000 MHz,

so 1.8 GHz is definitively UHF.

```
```>Specifically, any oscillator would need
negative impedance device, e.g., a bipolar
>transistor to sustain the oscillations.
So, what transistors would be used in e.g.,
>a GSM transmitter with a carrier frequency of 1.8 GHz?

Transistors have been much faster than that for a long time--I've built circuits that (accidentally) oscillated around 12 GHz, using 40-GHz SiGe bipolar transistors. You can get discretes with f_T values of 80 GHz or more  (BFP840 iirc).

Integrated transistors are getting near 1 THz, though not in silicon. A friend of mine from grad school, Mark Rodwell, has been doing great things in that line for some years now out at UCSD.

Before transistors were that good, there were klystrons, magnetrons, and travelling-wave tubes (TWTs), all of which still have their uses. For powers of milliwatts to watts, there are negative-resistance diodes such as Gunn and IMPATT (impact-avalance transit time), and for low power, high-order frequency multipliers such as step-recovery diodes running off lower frequency oscillators.

Now that frequency-locked femtosecond lasers are so good, you can make essentially any frequency you like by picking two teeth of the comb and beating them together, e.g. with a nonlinear crystal or a fast photodetector such as an antenna-coupled tunnel junction.

Cheers

Phil Hobbs
```
```On Tue, 27 Dec 2016 04:41:06 -0800 (PST), Phil Hobbs
<pcdhobbs@gmail.com> wrote:

>>So, what transistors would be used in e.g.,
>>a GSM transmitter with a carrier frequency of 1.8 GHz?
>
>Transistors have been much faster than that for a long time--I've built circuits that (accidentally) oscillated around 12 GHz, using 40-GHz SiGe bipolar transistors. You can get discretes with f_T values of 80 GHz or more  (BFP840 iirc).

To the original poster, one should remember that the f_T only define
the frequency at which _current_ gain drops to unity.

However, you might still have voltage gain and hence _power_ gain in
common base configuration above f_T, so do not be surprised, if some
transistor stage oscillates above f_T.

```
```On 27.12.16 08:21, dakupoto@gmail.com wrote:
> Could some electronics guru please shed some light on this ?
>
> How is the carrier wave for high frequency
> wireless communication transmitters generated ?
> Specifically, any oscillator would need
> negative impedance device, e.g., a bipolar
> transistor to sustain the oscillations.
> So, what transistors would be used in e.g.,
> a GSM transmitter with a carrier frequency of 1.8
> GHz
>

There are special components for UHF oscillators,
e.g. the magnetrons (2.4 GHz) in microwave ovens.

A free-running oscillator does not usually have
good enough stability for transmitting use.

oscillator at a lower frequency (some MHz to 100 MHz),
and multiplying the frequency to the destination
frequency range.

For complicated modulation (like cell-phones), the
multiplied frequency in mixed in an image-rejection
mixer with the modulation baseband information to
reach the final modulated signal.

--

-TV

```
```On 12/27/2016 08:25 AM, upsidedown@downunder.com wrote:
> On Tue, 27 Dec 2016 04:41:06 -0800 (PST), Phil Hobbs
> <pcdhobbs@gmail.com> wrote:
>
>>> So, what transistors would be used in e.g.,
>>> a GSM transmitter with a carrier frequency of 1.8 GHz?
>>
>> Transistors have been much faster than that for a long time--I've built circuits that (accidentally) oscillated around 12 GHz, using 40-GHz SiGe bipolar transistors. You can get discretes with f_T values of 80 GHz or more  (BFP840 iirc).
>
> To the original poster, one should remember that the f_T only define
> the frequency at which _current_ gain drops to unity.
>
> However, you might still have voltage gain and hence _power_ gain in
> common base configuration above f_T, so do not be surprised, if some
> transistor stage oscillates above f_T.

Theoretically, but I've never seen one do that by accident.  It's
generally hard to manage even f_T/2 in a purpose-built oscillator unless
you really breathe on it.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
```
```On Wednesday, December 28, 2016 at 12:45:03 AM UTC+11, Tauno Voipio wrote:
> On 27.12.16 08:21, dakupoto@gmail.com wrote:
> > Could some electronics guru please shed some light on this ?
> >
> > How is the carrier wave for high frequency
> > wireless communication transmitters generated ?
> > Specifically, any oscillator would need
> > negative impedance device, e.g., a bipolar
> > transistor to sustain the oscillations.
> > So, what transistors would be used in e.g.,
> > a GSM transmitter with a carrier frequency of 1.8
> > GHz
> >
>
>
> There are special components for UHF oscillators,
> e.g. the magnetrons (2.4 GHz) in microwave ovens.
>
> A free-running oscillator does not usually have
> good enough stability for transmitting use.
>
> oscillator at a lower frequency (some MHz to 100 MHz),
> and multiplying the frequency to the destination
> frequency range.

You can get etched silica crystals with a fundamental mode up to at least 315MHz

http://www.golledge.com/docs/products/xtl_sm/gsx758.htm

Connor-Winfield offer oscillators that go a bit higher

http://www.conwin.com/oscillators-clocks.html

When I was looking at packaged stable, low-jitter non-multiplied 500MHz oscillator a few years ago, it wasn't cheap (at around \$100) but it did make life easier.

--
Bill Sloman, Sydney
```
```On Mon, 26 Dec 2016 22:21:15 -0800 (PST), dakupoto@gmail.com wrote:

>Could some electronics guru please shed some light on this ?
>
>How is the carrier wave for high frequency
>wireless communication transmitters generated ?
>Specifically, any oscillator would need
>negative impedance device, e.g., a bipolar
>transistor to sustain the oscillations.
>So, what transistors would be used in e.g.,
>a GSM transmitter with a carrier frequency of 1.8
>GHz
>
>

oscillator (which is very stable) and control an oscillator of some
sort with a phase-locked loop, PLL.

The final oscillator is generally a fast transistor, a bipolar or
PHEMT, or for low power like a cell phone, lately just part of a CMOS
chip.

There are tons of variations. One can also amplify the harmonics of a
lower-frequency oscillator, in multiple stages of 2:1 or maybe 3:1.

See wikipedia for details.

--

John Larkin         Highland Technology, Inc

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

```