how to run a vga simulation using hspice

[Friendcheng]

hello everyone
i want to do a vga(variable gain amplifier) simulation about vctrl versus the gain in a fixed frequency using hspice.
but i don't know the command sentence
can anyone tell me the sentence
Thanks in advance,
Regards,

 

[Blackuni]

Hi,

define a transient source to supply the stimulus to u r VGA
Use transient simulation with sweep option to vary control voltage.
use arithmetic operation to calculate gain
************ sample**
vinp in gnd dc=val sin(0 1 10meg)
vontrol cont gnd 1

.tran 1n 100u sweep cont 1 3.3 0.1
.probe gain=par('(vout)/vin')
.
.
***********

Let me know if its not clear

Thanks,

 

[Friendcheng]

thank u at first

i have done the simulation according to your lead
and i have the value of the gain
but whereafter i have a new question
how can i see the waveforms about the contral voltage versus the gain

i use the spice explorer,and i attempt to change the x axis to the contral voltage,
but the spice explorer told me that "sweeping parameter not allowed as x_axis"

how can i change the x axis and see the waveforms about the vctrl vs the gain

looking forward to your reply
thankyou

 

[Blackuni]

Hi,

yes, its not possible with it. U can try with cosmosscope.

sorry for tht vague answer.

Thanks,

 

[Friendcheng]

Thank you for your reply.
My VGA is full differential architecture , and according to you sample ,I write sentence like this:
Vinp 2 0 dc=1.2v ac sin(0 0.5m 100meg)
Vinn 9 0 dc=1.2v ac sin(0 -0.5m 100meg)
Vctrl 4 0 dc=1.3v
And the outputs are connected to node 5 and 11 respectively
(the bandwidth is larger than 100meg for the result of vdb)
.op
.tran 10n 100n sweep vctrl 0.6 1.6 0.1
.probe gain=par('(v(11,5))/(v(2,9))')

After simulation , I change the x_axis to v(4), than I got a grotesquery waveform

1. according to fig(1),the gain is too high, it achieves 6E+24, but in vdb waveform, the gain is 7dB when the vctrl is 1.3v

2. according to fig(2),the gain is variable along with the time

MY question:
I attempt to adjust the command sentence, but it’s not effectual
Can you help me to solve the problem ?
Thank you