mc100el1648 vco , can't get it to work

[Yannick]

I made a pcb for the mc100el1648, i used the schematic described in
the datasheet to get a sine wave output (datasheet page 10 figure 13).
All components are smd, decoupling of every dc voltage with 68nF caps,
short tracks and opening in gnd plane around tank circuit to reduce
parasitic capacitance to gnd.

Tank circuit exist of 56nH coil (smd) and varicaps are 3-40pF range.
power supply betzeen 0 and 5V. Still i can't get it to work, the
output had dc shift of 3.3V but the ac signal is crap, it is very
noise, a lot of higher harmonics (ni nice sine wave) , pk-pk is 80mV
and frequency is about 60Mhz independed of varicap voltage. I tried
different loads on the output (from none to 50-100ohm termination) but
without a significant difference.

I am wondering i perhaps blew up the device by accidently putting one
of the varicaps in forward bias, is this possible or is the devide
robust enough in this regard?

Anyone an idea of what could be wrong here? Thanks,

Yannick

 

[Jim Thompson]

I made a pcb for the mc100el1648, i used the schematic described in
the datasheet to get a sine wave output (datasheet page 10 figure 13).
All components are smd, decoupling of every dc voltage with 68nF caps,
short tracks and opening in gnd plane around tank circuit to reduce
parasitic capacitance to gnd.

Tank circuit exist of 56nH coil (smd) and varicaps are 3-40pF range.
power supply betzeen 0 and 5V. Still i can't get it to work, the
output had dc shift of 3.3V but the ac signal is crap, it is very
noise, a lot of higher harmonics (ni nice sine wave) , pk-pk is 80mV
and frequency is about 60Mhz independed of varicap voltage. I tried
different loads on the output (from none to 50-100ohm termination) but
without a significant difference.

I am wondering i perhaps blew up the device by accidently putting one
of the varicaps in forward bias, is this possible or is the devide
robust enough in this regard?

Anyone an idea of what could be wrong here? Thanks,

Yannick

Double-check your connections.

What applied voltage do you have on the varicap? It must be above
~1.5V or the varicap will be forward-biased.

The MC1648 is one of my more popular designs ;-)

(MC100EL1648 just means it's on an Eclipse Lite Process.)

...Jim Thompson

 

[Yannick]

Double-check your connections.

What applied voltage do you have on the varicap? It must be above
~1.5V or the varicap will be forward-biased.

The MC1648 is one of my more popular designs ;-)

(MC100EL1648 just means it's on an Eclipse Lite Process.)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

America: Land of the Free, Because of the Brave- Tekst uit oorspronkelijk bericht niet weergeven -

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Ah you designed the original 1648, nice!!! So i actually double
checked connection 3 times, all voltages are in good range, indeed
varicap voltage is over 2V to be sure he is in reverse bias(i also
tried a fixed capacitor but didnt work either). I maybve blew up the
transistors of the diff pair from the tank input due too much current
(in the beginning i had accidently forward biassed the varicap) as you
are the expert and know how big these transistors W/L are , is it
possible i blew up the device ?

thanks,

Yannick

 

[Joerg]

Ah you designed the original 1648, nice!!! So i actually double
checked connection 3 times, all voltages are in good range, indeed
varicap voltage is over 2V to be sure he is in reverse bias(i also
tried a fixed capacitor but didnt work either). I maybve blew up the
transistors of the diff pair from the tank input due too much current
(in the beginning i had accidently forward biassed the varicap) as you
are the expert and know how big these transistors W/L are , is it
possible i blew up the device ?

Jim is on a biz trip and I am not too familiar with this chip. But did
you check the bias voltage at the inductor? AFAIR it should be a little
over 1.5V in a PECL circuit. If it's far from that the chip may be blown.

BTW, it is always good policy to apply a varicap voltage via a resistor.
Under normal operation there isn't any current.

 

[Jim Thompson]

Ah you designed the original 1648, nice!!! So i actually double
checked connection 3 times, all voltages are in good range, indeed
varicap voltage is over 2V to be sure he is in reverse bias(i also
tried a fixed capacitor but didnt work either). I maybve blew up the
transistors of the diff pair from the tank input due too much current
(in the beginning i had accidently forward biassed the varicap) as you
are the expert and know how big these transistors W/L are , is it
possible i blew up the device ?

thanks,

Yannick

The device emitter sizes were 25um X 37um in the original MC1648.

Now there probably 1.5um X 5um X M stripes... easy to blow.

...Jim Thompson

 

[Yannick]

The device emitter sizes were 25um X 37um in the original MC1648.

Now there probably 1.5um X 5um X M stripes... easy to blow.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

America: Land of the Free, Because of the Brave- Tekst uit oorspronkelijk bericht niet weergeven -

- Tekst uit oorspronkelijk bericht weergeven -

Ok, thanks ... i'l try a new device, it's a pity they are so expensive
in belgium !