Microwave, RF & LF Signal Sources & Sensor Systems

Spectral Line Systems Ltd

Research, Custom Design & Manufacture

Discriminator Stabilised Sources


Octave Band Low Phase Noise VCO
Photograph 1
0.5 to 1.0 GHz Discriminator Stabilised Source
Photograph 2
0.75 to 2.0 GHz Discriminator Stabilised Source
Photograph 3
15 GHz Delay Line Discriminator Stabilised Source
Photograph 4
Microwave Ultra-Low Phase Noise Source Module
Photograph 5

Discriminator Stabilised Sources consist of a voltage-controlled oscillator (VCO) stabilised by a frequency discriminator in a feedback loop from the source output to its tuning terminal. To obtain high loop gain and hence a high degree of phase noise reduction the source tuning slope and the discriminator sensitivity must both be as high as possible.


Spectral Line Systems Ltd Discriminator Stabilised Sources are based on a series of low noise, high tuning sensitivity and high output power VCO designs. These designs are also available packaged as stand-alone units, and can be supplied to individual customers' specifications. The model shown in photograph 1 is typical of octave-band units in the 500 MHz to 2.0 GHz range with +20 dBm of output power.


Frequency Discriminators are normally based on either delay line or resonator designs. Delay line discriminators tend to be used in the lower microwave region, and have the advantage of allowing the stabilised source to be tuned to any frequency over a wide operating bandwidth. This is done by including a suitable phase shifter in the feedback loop. Photograph 2 shows a 0.5 to 1.0 GHz source of this type. A similar source tuning over 0.75 to 2.0 GHz is shown in photograph 3. The latter source contains two VCO's to cover the wider frequency range.


Photograph 4 shows a Ku-Band source based on a 1.0 GHz delay line discriminator stabilised VCO followed by a X 15 SRD frequency multiplier and output amplifier. For good long term stability the 15.0 GHz signal can be locked to an internal or external crystal controlled reference.


Photograph 5 also shows a microwave source module, this time based on a resonator discriminator. The technology employed in an X-Band version of this type of source is capable of providing 10 to 15 dB lower phase noise than the best available crystal multiplier in the 10 KHz to 100 KHz offset from carrier region. Ku Band versions show a similar improvement.


In general, discriminator stabilised sources have lower far from carrier phase noise than crystal multiplier sources, at the expense of inferior close to carrier noise. Unlike crystal multiplier sources, however, a discriminator stabilised source can be tuned or frequency modulated over a relatively wide range from its design centre frequency.


A further advantage of discriminator stabilisation is that any power amplification following the source can be included within the feedback loop. This greatly reduces the added phase noise requirement of the power amplifier, which is often a problem when amplifying other types of low phase noise source to high power levels. Discriminator stabilised sources are therefore ideal for use with high power tubes, where both the tube and the drive amplifier can be included within the loop. Note that the effects of tube high voltage power supply ripple giving rise to FM spurious signals are also greatly reduced by including the tube within the loop.